J OURNAL OF Journal of Petrology, 2019, Vol. 60, No. 11, 2131–2168 doi: 10.1093/petrology/egaa002 P ETROLOGY Advance Access Publication Date: 11 April 2020 Original Article
Archean Boninite-like Rocks of the Northwestern Youanmi Terrane, Yilgarn Craton: Geochemistry and Genesis Jack R. Lowrey1,2, Derek A. Wyman 1*, Tim J. Ivanic2, Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021 R. Hugh Smithies2 and Roland Maas3
1School of Geosciences (F09), University of Sydney, Sydney, NSW 2006, Australia; 2Department of Mines and Petroleum, Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia; 3School of Earth Sciences, University of Melbourne, Melbourne, VIC 3010, Australia
*Corresponding author. School of Geosciences (F09), University of Sydney, Sydney, NSW 2006, Australia. Telephone: 61 2 9351 2924. E-mail: [email protected]
Received December 26, 2018; Accepted January 5, 2020
ABSTRACT Rocks with chemical compositions similar to Cenozoic boninites occur in many Archean cratons (boninite-like rocks), but they are rarely well-preserved, well-sampled, or presented within chrono- and chemo-stratigraphic context. This study provides a detailed description of the most extensive and well-preserved Archean boninite-like rocks reported to date. Within the 2820 to 2740 Ma mag- matic suites of the northwest Youanmi Terrane, Yilgarn Craton, boninite-like rocks occur as two distinct units. The first boninite-like unit is thinner (several 10 s of m thick), occurs close to the base of the 2820–2800 Ma Norie Group and includes both volcanic flows and subvolcanic intrusions. The second boninite-like unit is thicker (locally several 100 s m), occurs near the base of the 2800– 2740 Ma Polelle Group and consists of mainly fine-grained volcanic flows with local cumulate units. On average, major and trace element compositions for Youanmi Terrane boninite-like rocks are marginal between basalt, picrite and boninite and they have asymmetrically concave REE patterns, and Th–, Zr–Hf enrichments, similar to many Phanerozoic low-Si boninite suites, but at generally higher MREE–HREE contents. We report over 300 new whole-rock geochemical analyses, and 16 new Sm–Nd isotopic analyses, and associated petrographic evidence, including representative mineral compositions, which we support with published geochemical analyses and several deca- des of fieldwork in our study area. Comparison between Archean boninite-like rocks and Cenozoic boninites shows that most Archean examples had less depleted sources. We consider two possible petrogenetic models for the Youanmi Terrain examples: (1) they reflect variably contaminated komatiites, or (2) they reflect melts of metasomatised refractory mantle, analogous to Phanerozoic boninites. Trace element modelling indicates that crustal contamination could potentially produce rocks with boninite-like compositions, but requires an Al-enriched komatiitic parent liquid, for
which there is no field evidence in our study area. Initial eNdT values in pre-2800 Ma rocks (eNdT -0 4 to þ1 2) are on average slightly higher than those in 2800–2733 Ma examples (eNdT -3 2toþ1 2), compatible with increasing mantle metasomatism involving recycling of 2950 Ma crust. Integration of trace element and Nd isotopic data demonstrates that significant direct crustal as- similation was restricted to felsic magmas. The Th–Nb and Ba–Th systematics of mafic- intermediate rocks reflect fluid- and sediment-derived processes in the mantle, with boninite-like examples being linked primarily to fluid metasomatism. We compare the well-preserved igneous textures and mineralogy of Youanmi Terrane boninite-like rocks with those of their Phanerozoic
counterparts, and based on studies of the latter, suggest that former had similarly hot, H2O-rich parent magmas. The association of boninite-like rocks in the Norie and Polelle Groups with coeval high-Mg andesites, sanukitoids and hydrous mafic intrusions of the Narndee Igneous Complex
VC The Author(s) 2020. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: [email protected] 2131 2132 Journal of Petrology, 2019, Vol. 60, No. 11 strongly suggests a metasomatised mantle source and subduction operating in the Yilgarn be- tween 2820 and 2730 Ma.
Key words: Archean boninite; Archean subduction; mantle metasomatism; crustal contamination
INTRODUCTION Like komatiites and komatiitic basalts, boninitic melts The beginning of modern-style plate tectonics is a sub- can have high-MgO (e.g. up to 20 wt % MgO; Walker & ject of much debate, and one of the most contested Cameron 1983), but incompatible lithophile elements arguments is whether some Archean greenstones are are much higher than in komatiites. For example, analogous to modern volcanic arcs, or that their similar- Cenozoic boninites typically have very characteristic Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021 ities are superficial (Barley et al., 1984, 1989; Campbell concave REE patterns, and Th/Nb ratios higher than & Hill 1988; Condie 2005a, 2005b; Smithies et al., 2005; those inferred for MORB-OIB mantle. However, similar B-dard 2006, 2018; Pearce 2008; Be´ dard et al., 2013; characteristics would be expected in komatiitic melts Wyman 2013). Many Archean greenstone sequences contaminated by felsic crust, and so differentiating contain volcanic rocks that are chemically similar to between boninite and contaminated komatiite can be modern arc rocks (Basalt–Andesite–Dacite–Rhyolite; difficult. This is particularly pertinent for Archean BADR), yet such chemical signatures could be superim- boninite-like and low-Ti basalt (LOTI) suites, which form posed on non-arc melts by crustal contamination or by a component of greenstone sequences in many cratons mixing with crustally-derived melts (Pearce, 2008; Said and have been cited as strong evidence for modern- & Kerrich, 2010; Barnes et al., 2012; Barnes & Van style subduction processes in the Archean (Fan & Kranendonk 2014). Kerrich, 1997; Kerrich et al., 1998; Boily & Dion, 2002; To assess whether arc-like characteristics in evolved Polat et al., 2002; Manikyamba et al., 2005; Smithies BADR rocks were indeed derived from subduction- 2002; Smithies et al., 2004, 2005; Wyman & Kerrich, metasomatised mantle, it is necessary to first con- 2012; Angerer et al., 2013; Turner et al., 2014). In many strain the composition of their inferred mantle source. Archean greenstone terranes, it is unclear whether vol- High-Mg mafic volcanic rocks (including boninites) are canism occurred in an oceanic or continental setting, particularly useful in this respect because their compo- because pre-existing crust cannot be identified. Under sitions are relatively close to those of their primary such circumstances, tectonic models must rely heavily melts. A potential disadvantage of using highly mag- on inferences from sedimentary provenance, inherited nesian samples to trace primary magma composition zircon geochronology and isotopic compositions, and is their intrinsically low concentrations of incompatible whole-rock geochemistry, with potentially ambiguous trace elements, which are easily modified by alteration results and conflicting interpretations (e.g. Arndt et al., and/or small amounts of crustal contamination. 2001 vs Smithies et al., 2004). The best-known examples of Phanerozoic boninites, Here we present new whole-rock chemical and Sm– such as those in the Cenozoic Izu- Ogasawara-Mariana Nd isotopic data and investigate the petrogenesis of arcs, or the Tonga Ridge, are exclusively found in sub- high-Mg mafic volcanic rocks from a 2820–2735 Ma vol- duction zone settings, where they are mainly associated cano–sedimentary sequence near Meekatharra, in the with the embryonic stages of subduction. Boninites in Youanmi Terrane of the Yilgarn Craton, Western ophiolites are commonly thought to be representative Australia. Previous work identified a volcanic unit with of oceanic crust that formed in such settings (e.g. boninite-like compositions within this sequence Crawford et al., 1989; Pearce & Robinson, 2010; Haase (Wyman & Kerrich, 2012) and this study confirms that et al., 2015). Typically regarded as products of mantle boninite-like rocks are both laterally extensive and flux-melting, some authors have proposed that bonin- more representative of the Meekatharra Formation than ites, like komatiites, may additionally require anomal- previously appreciated. In addition, we describe a ously high melting temperatures, and have suggested newly identified occurrence of boninite-like rocks that upwelling hot refractory mantle plume-material is formed during the early stages of magmatism at drawn into the mantle wedge where it interacts with hy- c.2820 Ma. Unlike most Archean boninite-like rocks drous fluids and partial melts released from subducting described to date, the examples in the northwestern crust (e.g. Taylor et al., 1994; Portnyagin et al., 1997; Youanmi Terrane are remarkably well-preserved (low Macpherson & Hall 2001; Falloon et al., 2008; grade, greenschist facies metamorphism in some) and Kanayama et al., 2012). A similar model has also been retain much of their primary igneous mineralogy. This proposed for ultra-depleted Al-enriched komatiites in allows the rocks to be placed within a chemostrati- the Commondale greenstone belt, South Africa (Wilson, graphic framework that can be used for comparisons 2003a, 2003b), which plot as boninites in the new with modern volcanic settings. The new results, com- scheme of Pearce & Reagan (2019). bined with existing chemical and isotopic data from the Journal of Petrology, 2019, Vol. 60, No. 11 2133 western Youanmi Terrane, are used to further assess assigned to the Singleton Formation, overlain by a se- the petrogenesis of these magmas against the compet- quence of intermediate volcanic and sedimentary units, ing crustally-contaminated komatiite and embryonic including jaspilitic banded iron formations, cherts, and subduction models. shales, all assigned to the Yaloginda Formation. The mafic–ultramafic metavolcanic rocks of the Singleton Formation (now mostly talc–chlorite–tremo- GEOLOGICAL BACKGROUND lite–serpentine schists) have been interpreted by most The chronostratigraphic and magmatic framework of previous studies to have a komatiitic association the northwestern Youanmi Terrane (northern (Jackson, 1990; Reudavey, 1990, Watkins & Hickman, Murchison Domain) was recently revised by Van 1990; Barley et al., 2000; Hallberg, 2000; Pidgeon & Kranendonk et al. (2013). Archean supracrustal rocks Hallberg, 2000; Van Kranendonk et al., 2013). Barley are defined as the Murchison Supergroup, which is div- et al. (2000) invoked mantle plume ascent beneath Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021 ided into three main stratigraphic successions: (1) most- subduction-modified lithosphere to account for their ly felsic volcanic and sedimentary rocks from 2 98– trace element signatures and high volatile content, as 2 92 Ga; (2) voluminous (ultra)mafic to felsic volcanic reflected by pyroclastic occurrences. Wyman (2019), and (volcano-) sedimentary rocks (Norie Group, c.2820– however, argued that the rocks were similar to picrites 2800 Ma; Polelle Group, 2800–2735 Ma); and (3) silici- from Phanerozoic ophiolites and that pyroclastic occur- clastic and mafic volcanic rocks (Glen Group, c.2735– rences were the product of magma mixing with wet 2710 Ma), which unconformably overlie the Polelle crustal melts. Group. Two main geodynamic models are proposed for Mafic intrusions of the 2820–2815 Ma Meeline Suite, volcanism between c.2820 and 2735 Ma, including rift- including the Lady Alma Igneous Complex in the ing and mantle upwelling, leading to autochthonous Gabanintha region (Figs 2 and 3) and the voluminous crustal growth (Watkins & Hickman, 1990; Ivanic et al., Windimurra and Youanmi Igneous Complexes (approxi- 2012; Van Kranendonk et al., 2013), and hydrous melt- mately 100 km south of the study area; Ivanic et al., ing in a subduction zone environment (Wyman, 2019) 2017), are broadly coeval with the Singleton Formation. or a combination of the two (Champion & Cassidy, The Singleton Formation is overlain by a c.2 km thick 2002; Wyman & Kerrich, 2012). The stratigraphic rela- sequence of intermediate to felsic volcanic rocks tionships of the Murchison Supergroup are summar- assigned to the Yaloginda Formation. U–Pb zircon dat- ised in Fig. 1. ing of these volcanic rocks in the study area yielded The field area for this work lies between the towns of ages ranging from c.2815 6 7 to 2806 6 4Ma (Wang, Cue and Meekatharra, where several geological map- 1998; Wingate et al., 2011). In this area, the Yaloginda ping and geochronological studies (Watkins & Formation consists of porphyritic basaltic andesites Hickman, 1990; Wang et al., 1998; Hallberg, 2000; near its base, overlain by porphyritic and volcaniclastic Pidgeon & Hallberg, 2000; Romano, 2018) have estab- (medium–coarse grained breccia) andesite to dacite lished the ages of the geological units (Fig. 2). Two and fine-grained rhyolitic tuffs. areas with well-exposed stratigraphic intervals were Numerous fine-grained interflow sediments (promin- studied in detail: ent ridges of jaspilitic banded iron formation, cherts, shales and siltstones) are locally intruded by mafic– 1. Outcrops in the Gabanintha Mining District (approxi- ultramafic sills. A c.1500 m thick unit of these sediments mately 40 km SE of Meekatharra; Figs 2 and 3) ex- marks the boundary between the Yaloginda Formation pose the transition between two chemically distinct and the overlying Polelle Group. The abundance of thick suites of mafic–ultramafic volcanic rocks in the ridges of finely laminated banded iron formation, shale Singleton Formation. Most rocks are metamor- and chert indicates an extended period (at least locally) phosed to greenschist facies, but preserve primary of volcanic quiescence between the Norie and Polelle characteristics such as vesicles and relict igneous Groups. crystal boundaries. 2. Outcrops in the Polelle Syncline (approximately 10 km SE of Meekatharra; Fig. 2) expose the upper Polelle Group Norie Group (Yaloginda Formation) to upper Polelle The Polelle Group in the study area consists of the basal Group (Greensleeves Formation). The rocks in this Meekatharra Formation and the overlying Greensleeves area experienced only low-grade metamorphic over- Formation (Fig. 1). The Polelle Group is exposed printing (prehnite-pumpellyite to lower greenschist throughout much of the northwestern Youanmi facies) and are generally well-preserved, retaining Terrane, but is best preserved in the Polelle Syncline both igneous textures and in many cases primary (Fig. 2). In this locality the Meekatharra Formation com- mineralogy. prises four discrete units (Fig. 1). The basal unit is the Lordy Basalt Member, which is dominated by high-Mg Norie Group basalt with coarse acicular pyroxene phenocrysts and, The Norie Group in the study area (Fig. 2) consists of a locally, pyroxene spinifex textures. This is overlain by basal sequence of mafic–ultramafic volcanic rocks the Bassetts Volcanic Member, a high-Mg mafic 2134 Journal of Petrology, 2019, Vol. 60, No. 11 Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021
Fig. 1. Stratigraphic relationships of the Murchison Supergroup (modified after Ivanic, 2016; see also Romano, 2018). volcanic unit with boninite-like chemical compositions The 2799 6 7 Ma Narndee Igneous Complex, 100 km and textural characteristics (see also Wyman & Kerrich, south of the study area, is intruded immediately prior to 2012) that typically contains abundant fine acicular pyr- or coincident with the Meekatharra Formation and oxene phenocrysts and, locally, orthopyroxene-rich cu- hosts similar volumes of high-Mg lithologies (Ivanic mulate layers. The Bassetts Volcanic Member is et al., 2015). Importantly, Narndee gabbros contain overlain by the Stockyard Basalt Member, which is abundant igneous hornblende, with mantle-like H–O dominated by lower-Mg tholeiitic basalt that is typically isotopic ratios, reflecting the presence of (locally) massive, but locally contains pillow structures and hydrated mantle at the time of the Meekatharra vesicles. The youngest member of the Meekatharra Formation high-Mg magmatic activity. Formation is the Bundle Volcanic Member, which is a The basal unit of the Greensleeves Formation, the high-Mg basalt to basaltic andesite unit that consists of Woolgra Andesite Member, consists of interbedded vol- a sequence of several flows, internally zoned into platy canic flows, fragmental units, and volcanogenic sedi- pyroxene spinifex-textured flow tops and olivine-rich mentary units, ranging in composition from basaltic cumulate bases (Lowrey et al., 2017). andesite to dacite. Flows and volcanic fragmental rocks Journal of Petrology, 2019, Vol. 60, No. 11 2135 Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021
Fig. 2. Interpreted bedrock geology map of the northwest Youanmi Terrane (i.e. north Murchison Domain), Yilgarn Craton (modi- fied after Lowrey et al., 2017). This map shows the distribution of supracrustal groups and co-genetic intrusive suites, available geo- chronological data, major structural features and significant localities. Interpretation is based on 1:100,000 mapping by the Geological Survey of Western Australia (GSWA), and supplemented by our interpretation of aeromagnetic and Landsat multispec- tral images. Coordinates are relative to GDA94/MGA Z50. Geochronology sample sources (Wang, 1998; Pidgeon & Hallberg, 2000; Geological Survey of Western Australia, 2018). Inset abbreviations: MD, Murchison Domain; SCD, Southern Cross Domain; YT, Youanmi Terrane; NT, Narryer Terrane; SWT, South West Terrane; EGS, Eastern Goldfields Superterrane. are locally amygdular and typically porphyritic, with eu- ages (2761–2734 Ma; Van Kranendonk et al., 2013) indi- hedral plagioclase, clinopyroxene and, locally, horn- cate a c.20 Ma period of apparently continuous felsic blende phenocrysts, which commonly form volcanism. Numerous hornblende-rich tonalite plutons glomerocrysts (Hallberg et al., 1976). U–Pb in zircon assigned to the Cullculli Suite (dominantly 2760– 2136 Journal of Petrology, 2019, Vol. 60, No. 11 Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021
Fig. 3. Interpreted bedrock geology map of Gabanintha Mining District (see Fig. 2 for regional context). This map shows the distri- bution of geochemical samples discussed in this section.
2740 Ma; Van Kranendonk et al., 2013) intrude the Norie and Polelle Groups within the Meekatharra-Mt greenstone stratigraphy and are broadly coeval with Magnet greenstone belt (Fig. 2). Most samples were col- this period of volcanism. lected from outcrops during this study (301 samples) with care taken to avoid weathered, altered, veined or highly metamorphosed rocks. The Polelle Group sam- ples (sub-greenschist to greenschist) are better pre- SAMPLE SELECTION AND ANALYTICAL served than those from the Norie Group (greenschist to PROCEDURES locally granulite facies). Whole rock major, minor and trace element The samples were crushed by jaw crusher, milled concentrations using a low-Cr steel mill and then analysed at three Our study is based on a large set of whole-rock chem- Western Australian laboratories; Australian Laboratory ical analyses for 445 samples of volcanic rocks from the Services (ALS), Intertek Genalysis Laboratory Services Journal of Petrology, 2019, Vol. 60, No. 11 2137
Pty Ltd (Genalysis), and Bureau Veritas (BV). In addition using Eichrom TRU- and LN-resin (Pin & Santos to the samples collected during this study, we also re- Zalduegui, 1997). Total blanks (0 1 ng Nd, 0 016 ng Sm) port compositions for an additional 144 samples col- were negligible compared to sample sizes and no blank lected by GSWA during a previous mapping campaign corrections were applied. All isotopic data were (Watkins & Hickman, 1990) that were crushed in a plate acquired on a Nu Plasma multi-collector ICP-MS, with jaw crusher, milled in a tungsten-carbide mill and ana- sample introduction via a Glass Expansion low-uptake lysed together with our samples by ALS and Genalysis. PFA nebulizer and Cetac Aridus desolvator. Major and minor elements (Si, Ti, Al, Cr, Fe, Mn, Mg, Instrumental mass bias for data collected in static mode Ca, Sr, Ba, Na, K, and P) were determined by X-ray was corrected by internal normalization to fluorescence spectrometry (ALS method ME-XRF26, 146Nd/145Nd ¼ 2 0719425 (equivalent to the more famil- Genalysis method FB1/XRF; BV method XRF202). For iar 146Nd/144Nd ¼ 0 7219; Vance & Thirlwall, 2002) and 152 147 this purpose, fused discs were prepared by fusing a Sm/ Sm ¼ 1 .78307, respectively, using the expo- Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021 1:10 sample-flux mix (LiBO2, LiB4O7 and LiNO3 flux) at nential law as part of an online iterative spike subtrac- 1025–1100 C (depending on laboratory). Loss on igni- tion/internal normalization procedure. 143Nd/144Nd in tion was determined by thermogravimetric analysis unknowns and quality controls was adjusted to a nom- (ALS method ME-GRA05, Genalysis method TGA, BV inal 143Nd/144Nd ¼ 0 511860 for the La Jolla Nd stand- method LOI-1000). ALS and Genalysis determined litho- ard, which was analysed every fourth run and yielded phile trace element concentrations by fusing the sample measured (mass bias-corrected) ratios in the ranges with a flux mix (LiBO2, LiB4O7) then dissolving in acid 0 511848–0 511871, 0 511862–0 511884 and 0 511919– and analysis by ICP-MS (for Cs, Rb, Ba, Sr, Th, U, Nb, 0 511969 in the three analytical sessions, respectively. Ta, Zr, Hf, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, After adjustment, the JNd-1 Nd standard averaged Tm, Yb and Lu at both laboratories, and Cr, V at ALS 0 512120 6 14 (2sd, n ¼ 6) while the USGS basalt BCR-2 only; ALS method ME-MS81, Genalysis method FB6/ yielded 147Sm/144Nd ¼ 0 1382 6 2 (2sd, n ¼ 4) and MS), or ICP-OES (Cr, V, Sc at Genalysis only, method 143Nd/144Nd ¼ 0 512625 6 17 (2sd, n ¼ 6); these results FB6/OE). ALS and Genalysis determined base metal are consistent with long-term averages and with TIMS/ concentrations by dissolving samples with a 4-acid mix- MC-ICP-MS reference numbers. External precisions for 147 144 143 144 ture (HClO4, HNO3, HF and HCl) followed by analysis via Sm/ Nd and Nd/ Nd are 60 2% and 60 004% ICP-AES (Co, Cu, Ni, Pb, Sc and Zn; ALS method (2sd), respectively. eNd values were calculated using 4ACD81), ICP-OES (Cu, Ni, and Zn; Genalysis method the CHUR parameters of Bouvier et al. (2008): 4 A/OE) or ICP-MS (Co and Pb; Genalysis method 4 A/ 147Sm/144Nd ¼ 0 1960, 143Nd/144Nd ¼ 0 512632* (*ad MS). BV determined lithophile element and base metal justed to the La Jolla value used here). 147Sm/144Nd and concentrations by laser ablation on the fused discs pre- 143Nd/144Nd in modern depleted mantle (DM) are 0 2136 pared for XRF (BV method LA101). and 0 513163, respectively, and are based on a linear Data reproducibility at the three laboratories is evolution from eNd ¼0at4 56 Ga to eNd¼þ10 at the broadly comparable. Total uncertainties for major ele- present (modified from Goldstein et al., 1984). The ments are 1 5%, those for minor elements are < 2 5% 147Sm decay constant is 6 54x10-12 yr1. Age corrections (at concentrations > 0 1 wt %) and those for trace ele- (eNdT) are based on U–Pb zircon ages where available ments are 10% (Lu 6 20%). Major elements are (see Table 2); otherwise, generic ages of 2820 Ma are re-calculated to anhydrous concentrations. Primitive used for Singleton Formation samples, 2815 Ma for mantle abundances are those of Sun & McDonough Yaloginda Formation samples and 2800 Ma for (1989). The elemental compositions of selected samples Meekatharra Formation samples. are presented in Table 1, while the complete set of Results are presented in Table 2 and are included in results, including sample locations, are included in digital format in Electronic Appendix 2 (together with Supplementary Data Electronic Appendix 1; supple- data published by the Geological Survey of Western mentary data are available for downloading at http:// Australia, 2019). Unpublished Nd isotopic data for 12 www.petrology.oxfordjournals.org. samples from similar aged geologic units in the south western Youanmi Terrane were obtained by identical methods and are compared to our data in the discus- Sm–Nd isotope analytical methodology sion section (‘Sm–Nd isotopic variation’). These will be Sm–Nd isotopic compositions for 16 samples were presented in a subsequent publication that presents acquired at the University of Melbourne, following them in their geological context. Maas et al. (2015) and Mole et al. (2018). Powders (c.0 1 g) were weighed into Krogh-type PTFE vessels and mixed with a 149Sm-150Nd tracer calibrated against RESULTS the Caltech Sm–Nd mixed normal solution (Wasserburg Petrography and mineral chemistry et al., 1981). Samples were dissolved at high pressure Petrographic descriptions are limited here to mafic vol- (2 5 ml 3:1 HF-HNO3, 48 h, 160 C; 2x dry-down with canic suites, which are the focus of the discussion sec- conc. HNO3;2 5 ml 6 M HCl, 24 h, 160 C); clear solutions tion below. Compositions for Meekatharra Formation were obtained in all cases. Sm and Nd were extracted pyroxenes and Cr-spinels are tabulated and discussed Table 1: Major, minor and selected trace-element data for representative samples of Norie and Polelle Group volcanic rocks 2138 Geological unit: Norie Group, lower Singleton Formation Norie Group, upper Norie Norie Norie Group, Singleton Formation Group, Group, Yaloginda Formation Quinns Yaloginda Basalt For-mation Lithology: Basalt picrite to Ol dolerite/gabbro basalt High HFSE tholeiitic basaltic andesite to andesite orthocumulate to Px cumulate (LOTI-boninite-like) basalt to rhyolite (high-Mg andesite)
Sample ID: 218818 221772 218820 221769 221770 221759 221754 221755 221758 221746 221752 217739 217748 227208 217765 217768 %(anhydrous) Al2O3 10 37 8 40 4 66 6 29 4 07 10 68 14 63 10 75 10 98 15 66 12 60 14 26 14 34 11 88 15 07 14 23 CaO 10 27 9 71 4 87 8 46 4 78 10 69 5 73 5 97 7 24 10 97 10 60 8 90 0 05 6 76 5 15 5 95 Fe2O3(Total Fe) 12 46 12 87 13 76 13 17 12 64 11 32 9 52 10 47 10 68 10 89 12 17 10 20 3 85 11 07 8 31 3 93 K2O0 29 0 04 0 03 0 04 0 02 0 54 1 50 0 31 0 07 0 31 0 27 0 25 0 68 0 62 2 01 0 12 MgO 10 32 16 00 28 78 23 49 31 62 14 01 13 85 19 06 17 87 7 54 10 51 6 40 4 14 11 73 6 72 5 40 MnO 0 20 0 19 0 15 0 16 0 20 0 18 0 18 0 18 0 19 0 16 0 21 0 14 0 01 0 14 0 12 0 07 Na2O2 21 1 62 0 21 0 77 0 16 0 92 1 86 0 87 0 87 2 03 1 55 2 36 0 51 1 78 3 72 7 34 P2O5 0 08 0 07 0 04 0 06 0 04 0 03 0 02 0 02 0 02 0 09 0 05 0 24 0 02 0 14 0 16 0 13 SiO2 52 68 50 24 46 92 46 83 45 95 51 32 52 63 52 28 51 91 51 63 51 48 56 02 76 10 55 09 58 02 62 17 TiO2 1 11 0 86 0 59 0 74 0 51 0 32 0 10 0 11 0 16 0 72 0 54 1 20 0 29 0 79 0 68 0 64 LOI 1 12 2 67 01 3 34 7 87 1 43 3 56 2 41 1 55 1 75 1 53 100 22 99 7 100 03 2 52 0 93 ppm Cs 0 14 0 20 10 20 20 20 68 0 48 0 06 0 10 20 10 15 0 10 57 0 18 Rb 9 40 51 80 81 16 637 65 91 77 6 85 518 411 446 12 9 Ba 70 893 7675 811 758 7 335 14 532 586 968 3 130 5 146 5 251 7 670 31 2 Th 0 75 0 30 21 0 20 10 4 bdl 0 07 0 39 1 0 81 99 8 12 23 23 5 43 U0 17 0 10 1 0 1 0 10 1 bdl 0 02 0 14 0 20 20 44 2 12 0 40 81 0 92 Nb 3 62 51 81 60 91 10 21 0 38 0 72 81 57 822 93 25 66 6 Sr 120 44 337 838 418 736 76836 855 9 128 163 6 115 25 9 293 7 212 31 1 Pb Bdl 0 93 0 50 8 bdl bdl bdl bdl 1 81 3 bdl 6 6 43 3 Hf 2 1 30 81 0 60 70 18 0 22 0 52 1 61 14 315 31 72 63 5 Zr 72 44 32 35 23 22 5 7 5 17 60 42 162 512 60 108 137 Y19 612 88 79 96 411 14 44 4 08 5 618 717 146 2 117 515 615 719 Ta 0 10 2 bdl 0 30 40 3 bdl 0 02 0 04 0 60 40 41 40 20 30 4 La 5 14 13 12 51 62 10 27 0 54 1 91 6 4 11237 910 316 517 Ce 12 89 56 5 33 14 10 46 1 22 4 68 12 27 626 59221 63033 8 Pr 1 91 1 30 97 0 90 60 70 07 0 19 0 62 1 51 3 92 11 72 93 43 4 02 Nd 9 47 34 64 83 13 60 33 0 86 2 52 7 45 817 652 412 212 615 Petrology of Journal Sm 2 85 1 91 37 1 81 0 80 09 0 24 0 51 81 44 85 14 95 2 92 64 3 35 Eu 1 01 0 60 46 0 60 40 30 07 0 10 21 0 60 51 56 4 16 0 60 81 0 9 Gd 3 28 2 61 53 2 1 31 40 24 0 34 0 72 42 16 75 16 75 3 2 74 3 06 Tb 0 56 0 40 20 30 20 20 06 0 07 0 10 40 41 18 2 86 0 50 38 0 41 Dy 3 41 2 31 48 2 21 11 90 50 59 0 82 92 88 01 18 25 2 62 48 2 63 Ho 0 66 0 50 29 0 40 30 40 16 0 16 0 19 0 60 71 74 4 10 60 49 0 57 Er 1 97 1 30 82 1 20 91 10 54 0 46 0 73 2 1 85 37 13 15 1 81 58 1 8
Tm 0 27 0 20 12 0 20 10 20 10 08 0 11 0 30 30 82 01 0 30 22 0 25 11 No. 60, Vol. 2019, , Yb 1 76 1 0 75 1 0 61 30 86 0 66 0 87 2 31 84 92 13 65 1 91 43 1 86 Lu 0 25 0 20 11 0 20 10 30 15 0 12 0 14 0 40 30 82 33 0 30 23 0 27 Ni 243 637 1680 1167 1652 675 366 684 802 145 313 87 2 268 120 154 Cr 810 2020 2710 2002 1945 2881 884 3170 2670 317 853 120 10 820 260 190 V 282 249 115 190 122 189 126 131 117 220 210 248 bdl 219 150 115 Co 61 74 161 74 108 69 61 80 76 44 57 34 bdl 48 31 13 Cu 14 101 10 20 8 9 bdl 4 bdl 148 10 55 3 78 1 6 Sc 40 35 24 23 15 31 35 31 30 32 34 32 3 24 19 15 Zn 70 96 94 57 93 63 35 50 45 64 84 74 53 80 58 18
(continued) Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021 September 27 on guest by https://academic.oup.com/petrology/article/60/11/2131/5700738 from Downloaded Table 1: Continued ora fPetrology of Journal Geological unit: Polelle Group, Meekatharra Polelle Group, Meekatharra Formation, Polelle Group, Meekatharra Formation, Lordy Basalt Member Bassetts Volcanic Member Formation, Stockyard Basalt Member
Lithology: basalt to basaltic andesite Ol basalt to basaltic andesite Ol-Px Opx Tholeiitic basalt Tholeiitic basalt to (siliceous high-Mg basalt) ortho-cumulate (siliceous high-Mg basalt) ortho-cumulate ortho-cumulate to basaltic andesite basaltic andesite (population 1) (population 2)
Sample ID 217712 217771 221627 217714 221693 209130 217798 209176 209198 217755 217757 217788 217791
% 11 No. 60, Vol. 2019, , (anhydrous) Al2O3 10 81 13 36 9 54 6 28 13 96 12 27 11 49 6 09 8 53 14 44 14 78 13 97 14 83 CaO 8 20 6 55 7 85 4 95 10 52 9 76 8 25 5 01 6 34 11 93 11 70 10 00 10 35 Fe2O3 (Total Fe) 12 73 10 94 12 39 12 88 9 56 10 19 10 28 10 91 9 93 10 11 10 71 14 34 11 25 K2O0 06 0 12 0 04 0 15 0 07 0 08 0 12 0 05 0 37 0 11 0 15 0 38 0 35 MgO 16 43 9 81 19 34 27 44 9 55 12 00 14 35 29 70 19 37 7 56 6 16 6 29 5 49 MnO 0 17 0 13 0 18 0 17 0 16 0 18 0 17 0 17 0 18 0 18 0 20 0 20 0 23 Na2O1 15 3 63 0 38 0 52 1 40 1 15 1 25 0 14 0 69 2 08 1 87 2 05 2 51 P2O5 0 06 0 07 0 05 0 03 0 06 0 05 0 04 0 02 0 04 0 06 0 07 0 13 0 14 SiO2 49 54 54 56 49 32 46 78 54 12 53 69 53 41 47 20 53 94 52 74 53 48 51 25 53 50 TiO2 0 63 0 76 0 58 0 35 0 55 0 46 0 46 0 23 0 35 0 75 0 85 1 39 1 34 LOI 4 42 2 79 4 95 6 97 1 98 3 13 2 63 6 01 3 37 0 93 0 899 93 99 06 ppm Cs 0 10 10 12 0 78 0 18 1 30 25 0 53 4 0 06 0 06 0 09 0 1 Rb 1 43 6251 72 53 63 724 31 42 21211 4 Ba 32 49 828 12930 622 3 194 522 427 749 697 179 977 2 Th 0 82 1 38 0 80 43 0 68 0 69 0 65 0 24 0 50 22 0 28 0 46 0 44 U0 22 0 36 0 28 0 08 0 22 0 19 0 17 0 05 0 1 bdl 0 07 0 12 0 09 Nb 1 82 21 60 91 1 10 60 30 51 71 93 3 2 Sr 57 146 79 61732 137 532 95 826 389 7 131 5 126 101 5 Pb bdl bdl bdl bdl 5 4 bdl 4 0 5 bdl bdl bdl bdl Hf 1 31 71 20 71 21 0 90 40 81 11 62 32 4 Zr 44 65 45 23 44 38 36 15 21 44 57 89 88 Y15 517 613 77 814 815 213 56 9 516 819 429 527 Ta bdl 0 10 1 bdl 0 1 bdl bdl bdl bdl bdl 0 10 10 1 La 3 94 93 21 92 72 72 41 11 42 32 84 34 3 Ce 8 511 27 84 15 65 54 92 13 36 27 912 111 8 Pr 1 09 1 48 1 01 0 58 0 73 0 70 61 0 30 41 01 1 31 91 1 94 Nd 5 36 64 82 53 53 32 91 31 85 16 49 810 1 Sm 1 48 2 03 1 42 0 74 1 19 1 11 0 97 0 38 0 71 56 2 14 3 29 3 24 Eu 0 55 0 57 0 54 0 27 0 47 0 42 0 46 0 16 0 30 68 0 82 1 15 1 37 Gd 1 99 2 59 1 86 1 2 17 1 89 1 65 0 75 1 42 54 2 99 4 43 4 35 Tb 0 39 0 47 0 35 0 18 0 40 36 0 36 0 14 0 20 41 0 51 0 80 75 Dy 2 56 3 27 2 51 1 28 2 57 2 52 35 0 95 1 42 86 3 38 5 55 03 Ho 0 54 0 67 0 50 26 0 55 0 58 0 55 0 21 0 30 63 0 76 1 13 1 05 Er 1 67 2 1 54 0 81 88 1 68 1 49 0 79 1 12 03 2 13 43 12 Tm 0 22 0 30 23 0 13 0 29 0 26 0 25 0 11 0 20 28 0 34 0 57 0 47 Yb 1 53 1 93 1 34 0 77 1 73 1 73 1 50 71 1 31 86 2 01 3 36 3 09 Lu 0 23 0 30 23 0 13 0 25 0 26 0 24 0 13 0 20 26 0 34 0 50 45 Ni 521 106 761 1200 175 280 360 1010 357 106 118 120 102 Cr 1310 420 2450 2640 420 1150 1340 3200 1798 250 210 110 110 V 217 263 217 86 303 235 230 120 181 285 306 368 355 Co 72 42 78 119 44 48 53 87 55 35 41 54 42 Cu 98 89 21 43 53 47 43 23 29 21 9 191 193 2139 Sc 31 40 29 20 36 33 33 19 33 38 38 39 37 Zn 79 78 94 79 65 61 68 63 58 58 67 106 109
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Geological unit: Polelle Group, Meekatharra Formation, Polelle Group, Meekatharra Polelle Group, Bundle Volcanic Member Formation, Cue basalt (informal) Greensleeves Formation
Lithology: basalt to basaltic andesite Ol–Px ortho-cumulate Depleted tholeiitic basalt basaltic andesite to rhyolite (siliceous high-Mg basalt)
Sample ID 209114 209118 209123 217800 218803 221739 217722 227211 % (anhydrous) Al2O3 10 57 9 03 5 04 14 18 14 12 13 24 12 48 12 94 CaO 6 71 10 16 4 73 11 87 11 96 11 91 5 68 4 29 Fe2O3 (Total Fe) 10 53 11 40 11 14 12 70 13 82 9 16 6 45 3 29 K2O0 20 0 21 0 09 0 12 0 14 1 14 2 84 2 16 MgO 11 31 15 85 31 04 6 51 5 81 5 19 4 40 1 29 MnO 0 15 0 20 0 16 0 24 0 24 0 29 0 07 0 08 Na2O2 42 1 38 0 16 2 48 2 48 3 05 3 63 3 38 P2O5 0 06 0 06 0 04 0 06 0 06 0 19 0 15 0 15 SiO2 57 19 50 89 46 76 51 00 50 55 55 24 63 34 71 94 TiO2 0 66 0 62 0 34 0 80 0 80 0 55 0 93 0 45 LOI 2 56 3 26 80 55 0 44 8 11 2 11 71 ppm Cs 0 55 0 41 89 0 01 0 03 0 40 41 0 6 Rb 5 15 7 21 71 125 765 339 3 Ba 75 897 234 5 187 39 7 244 4 489 929 3 Th 1 44 1 40 80 33 0 34 42 97 10 4 U0 43 0 40 23 0 06 0 08 1 0 67 2 5 Nb 2 72 51 50 80 73 65 75 2 Sr 106 50 217 270 771 7 354 169 1 378 6 Pb 4 1 1 5 bdl bdl 5 83 10 8 Hf 1 41 30 71 41 32 72 23 1 Zr 54 50 29 47 45 101 82 115 Y14 813 77 726 826 412 213 111 3 Ta bdl 0 2 bdl bdl bdl 0 40 20 3 La673 32 51 819 11123 7 Ce 12 714 66 85 84 937 52142 2 Pr 1 62 1 80 85 0 91 0 81 4 22 65 4 3
Nd 7 57 73 94 84 315 810 216 1 Petrology of Journal Sm 2 02 1 21 1 76 1 63 2 92 68 2 8 Eu 0 64 0 60 34 0 70 55 0 80 78 0 8 Gd 2 41 2 31 19 3 03 2 84 2 32 61 2 3 Tb 0 41 0 40 21 0 61 0 55 0 40 35 0 3 Dy 2 64 2 61 34 36 4 21 2 12 18 1 6 Ho 0 55 0 50 28 0 93 0 98 0 50 45 0 4 Er 1 56 1 40 79 2 97 2 98 1 31 29 1 2 Tm 0 23 0 2012 0 5048 0 2018 0 2
11 No. 60, Vol. 2019, , Yb 1 49 1 40 79 3 23 3 05 1 11 08 1 1 Lu 0 23 0 20 12 0 48 0 48 0 20 16 0 1 Ni 230 318 1290 80 87 157 112 52 Cr 1470 1417 3540 160 140 272 140 192 V 177 195 111 329 309 108 110 84 Co 56 53 90 51 55 24 22 10 Cu 75 63 37 46 4 40 50 337 Sc 29 30 15 46 47 13 15 bdl
Zn 84 68 65 74 69 66 27 36 Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021 September 27 on guest by https://academic.oup.com/petrology/article/60/11/2131/5700738 from Downloaded Journal of Petrology, 2019, Vol. 60, No. 11 2141
Table 2: Isotopic data for Norie and Polelle Group volcanic rocks
Sample ID Group Formation Member Age (T) Sm Nd 143Nd/144Nd error 147Sm /144Nd eNdT T2DM (GA) (ppm) (ppm) (standard (2se) corrected)
221772 Norie Singleton lower 2820 2 17 30 512233 0 000014 0 171125 1 23 03 221754 Norie Singleton upper 2820 0 10 40 512848 0 000046 0 201898 2 02 96 221755 Norie Singleton upper 2820 0 20 80 512332 0 000016 0 179153 0 23 10 221768 Norie Singleton upper 2820 1 34 10 512410 0 000011 0 182926 0 33 09 221757 Norie Singleton upper 2820 0 93 00 512324 0 000008 0 177419 0 63 06 221797 Norie Singleton upper 2820 1 76 20 512131 0 000010 0 168017 0 33 09 221747 Norie Singleton upper 2820 2 07 80 511905 0 000009 0 154109 0 93 04 217768 Norie Yaloginda 2820 3 216 50 511174 0 000012 0 116769 0 23 09 227204 Norie Yaloginda 2800 2 08 00 511741 0 000008 0 148014 0 13 11 209130 Polelle Meekatharra Bassetts 2800 1 13 10 512756 0 000013 0 203760 0 53 13 Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021 217797 Polelle Meekatharra Bassetts 2800 1 02 90 512825 0 000012 0 207501 0 53 13 209114 Polelle Meekatharra Bundle 2800 2 07 60 511995 0 000010 0 160856 0 13 08 217712 Polelle Meekatharra Lordy 2800 1 75 60 512338 0 000013 0 179573 0 13 09 217771 Polelle Meekatharra Lordy 2800 2 06 70 512264 0 000012 0 176330 0 23 11 217788 Polelle Meekatharra Stockyard 2800 2 88 60 512715 0 000010 0 196856 1 23 00 217790 Polelle Meekatharra Stockyard 2800 4 213 50 512537 0 000010 0 188383 0 83 04
in more detail in Supplementary Data Electronic pyroxene spinifex textured tops (Fig. 6e; see also Appendix 3. Lowrey et al., 2017), and olivine–pyroxene cumulate bases (Fig. 6f). Olivine is typically serpentinized, while orthopyroxene and augite are variably preserved. Singleton Formation Vitrophyric flows locally contain globular ‘varioles’ of Lower Singleton Formation samples vary between fine- devitrified glass and variably altered acicular pyroxene grained mafic volcanic rocks (locally amygdular; Fig. 4a microphenocrysts (Fig. 7). and b) and olivine–pyroxene cumulates with varying Samples from the Bassetts Volcanic Member are typ- olivine abundances (Fig. 4c–f). Igneous minerals have ically fine-grained mafic volcanic rocks, with a glassy typically been replaced by greenschist–amphibolite (vitrophyric) or microlitic groundmass (Fig. 8a–f), metamorphic assemblages: olivine is typically altered reflecting high cooling rates and/or magma undercool- to serpentine; pyroxene is typically altered to actinolite ing linked to rapid fluid loss from water-saturated mag- and chlorite; and plagioclase is typically saussuritized. mas during ascent (Crabtree & Lange, 2011). Locally, textures are well preserved, including skeletal Vitrophyric rocks commonly contain globular varioles crystal features (Fig. 4d). ( 2 cm; Fig. 7), composed acicular pyroxene microphe- Higher in the Formation, textural preservation is nocrysts and devitrified glass that locally merge to- quite variable, due locally to amphibolite–hornfels fa- gether to form discrete domains of leucocratic cies metamorphism, but where textures are preserved groundmass ( 10 cm; e.g. Fig. 7b). All fine-grained (at greenschist facies), volcanic rocks vary from aphyric samples contain abundant, randomly oriented micro- to intensely plagioclase–pyroxene phyric or glomero- phenocrysts that are compositionally zoned, with phyric (Fig. 5a and b). Outcropping together with the pigeonite to clinoenstatite compositions in cores volcanic rocks are fine-grained subvolcanic units, typic- (Wo En Fs to Wo En Fs ; Supplementary Data ally with sub-ophitic textures dominated by stubby to 4 83 13 15 68 17 Electronic Appendix 3, Supplementary Data Fig S3.2) acicular zoned pyroxene pseudomorphs and albitized that are abruptly separated from rims with augite com- feldspar 6 Cr-spinel (Fig. 5c and d) Both volcanic and positions (mostly Wo En Fs to Wo En Fs ; subvolcanic rocks locally contain segregated amorph- 30 56 14 42 44 14 Supplementary Data Electronic Appendix 3, ous domains of leucocratic material with cuspate mar- Supplementary Data Fig S3.2). Gradational elemental gins (now altered to actinolite–tremolite–chlorite) within zonation (Fe, Mg, Ca, Ti, Al and Cr) is also observed a relatively homogenous mafic groundmass, which within discrete cores and rims, as shown in detailed may reflect mixing between two melts or partially element maps of a representative zoned clinopyroxene resorbed material that crystallized earlier in shallow- phenocryst (discussed further in Supplementary Data level magma chambers (Fig. 5e and f). Electronic Appendix 3 and shown in Supplementary Data Fig S3.3). Orthopyroxene is present as a pheno- Meekatharra Formation cryst phase in fine grained samples, but is typically The Lordy Basalt Member is characterized by coarse (1– altered to chlorite 6 serpentine minerals. 10 cm), randomly oriented, acicular zoned clinopyrox- Cr-spinel is present as a minor phase (< 5%), typical- ene (augite) phenocrysts within a pyroxene ly in clusters of fine (10–50 mm) euhedral phenocrysts microphenocryst-rich, or glassy groundmass (Fig. 6a– with an irregular distribution. Cr-spinels contain high d). Locally, flows have developed spectacular platy Cr2O3 (47 2–53 7 wt %), high Cr/(CrþAl) (0 82–0 85), low 2142 Journal of Petrology, 2019, Vol. 60, No. 11 Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021
Fig. 4. Lower Singleton Formation photomicrographs showing a variety of cooling histories, mineralogy and textures. (a) Fine- grained high-Mg basalt containing abundant pseudomorphed acicular pyroxene phenocrysts in a groundmass of devitrified, altered glass (sample 221772, 16 wt % MgO); (b) microlite pseudomorphs (probably plagioclase originally) in devitrified, altered glass (221772); (c) picrite containing abundant pseudomorphed olivine phenocrysts in a glassy groundmass, with coarser pheno- cryst now pseudomorphed by tremolite (sample 9344 of Hallberg, 2000; 24 wt % MgO); (d) olivine crystal morphologies in sample 9344 include discrete phenocrysts or glomerocrysts and locally skeletal forms; (e) and (f) PPL and XPL images, respectively, of orthocumulate with polyhedral olivine and poikilitic pyroxene pseudomorphs, now altered to serpentine and tremolite (sample 218822, 28 wt % MgO).
Fe3þ/(Fe3þþCrþAl) (0 09–0 11), high Fe2þ/(MgþFe2þ) and equant (150–500 mm), polyhedral or sub-angular (0 58–0 87) and low TiO2 (0 2–0 3 wt %), plotting entirely and typically serpentinized (not shown). within the 90th percentile (and mostly within the 50th Samples from the Stockyard Basalt Member are typ- percentile) contours for both boninite and komatiite on ically aphyric and fine-grained, with a groundmass Barnes & Roeder’s (2001) trivalent ion ternary plot. comprising predominantly augite, plagioclase and opa- Plagioclase is absent from vitrophyric samples, but is que minerals (e.g. magnetite, titanomagnetite; Fig. 9a present in slower cooled examples as lath shaped and b). Compositions of augites are more iron-rich than microlites (locally fan shaped spherulites) in a fine in the other Member units of the Meekatharra groundmass (Fig. 8c and d). Olivine has not been Formation (Supplementary Data Fig S3.1 in Electronic observed in any fine-grained examples; however, sev- Appendix 3). eral flows include 5–10 m thick orthocumulate layers Samples from the Bundle Volcanic Member are typ- containing cumulus orthopyroxene 6 olivine with fi- ically pyroxene spinifex-textured, or olivine-pyroxene brous intergrowths of augite, plagioclase, and opaque orthocumulates. The chilled margins of flows are rarely minerals in the interstices between orthopyroxene phe- preserved, but those that outcrop are aphyric, with skel- nocrysts (Fig. 8g and f). Orthopyroxene phenocrysts etal olivine microphenocrysts and dendritic clinopyrox-
(Wo3En85Fs12 to Wo3En84Fs13; Supplementary Data ene. The petrography of the Bundle Volcanic Member is Electronic Appendix 3) in orthocumulate zones are typ- described in detail in Lowrey et al. (2017). ically fresh, coarse, and elongate or columnar ( 5x Basalts from the Cue area (Fig. 2) typically have 1 mm), with locally developed epitaxial rims of augite. microlitic to sub-ophitic textures and have been sub- Olivine phenocrysts in orthocumulate zones are fine jected to upper greenschist–lower amphibolite facies Journal of Petrology, 2019, Vol. 60, No. 11 2143 Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021
Fig. 5. Upper Singleton Formation photomicrographs. (a) porphyritic basaltic andesite with plagioclase and pyroxene phenocrysts in fine-grained groundmass (sample 221799, 8 wt % MgO); (b) pyroxene–plagioclase glomerocryst set in a fine-grained ground- mass containing abundant plagioclase microphenocrysts (221799); (c) pyroxene-rich basalt with acicular phenocryst pseudo- morphs set in a trachytic textured groundmass (sample 221754, 14 wt % MgO); (d) compositional zoning in pyroxene pseudomorph (sample 81573, 16 wt % MgO); (e) and (f) amorphous mafic globules within a discrete zone of leucocratic groundmass adjacent to more mafic groundmass (sample 221793, 6 wt % MgO). metamorphism. The microlitic-textured rocks have earth elements (REE), Cr, Ni). X–Y variation diagrams pseudomorphed microphenocrysts of plagioclase for many of these elements are shown in Fig. 10a–h. We (saussuritized) and/or pyroxene (now typically actino- describe the chemical and isotopic variation within suc- lite–tremolite) characteristic of rapid cooling. The cessive stratigraphic intervals of the Norie and Polelle groundmass typically comprises lath-shaped plagio- Groups below and compare key chemical clase microlites with clinopyroxene pseudomorphs fill- attributes to well-known examples of boninite and ko- ing the spaces between laths (Fig. 9c and d). Locally matiite in Fig. 11a–f. We also present a summary of they contain larger euhedral pyroxene 6 plagioclase chemical variation within stratigraphic units of the glomerocryst pseudomorphs that have been entirely Norie and Polelle Groups in Table 3. replaced by ultrafine-grained alteration minerals. Locally, microlitic textures grade into coarser, almost Singleton Formation sub-ophitic textures, with coarse laths of plagioclase The Singleton Formation in the Gabanintha area (Figs (saussuritized), acicular pyroxene pseudomorphs (now 1–3) can be separated into two distinct geochemical actinolite–tremolite) and almost no fine-grained sub-units, corresponding to the lower part and upper groundmass, reflecting variable cooling histories. part of the formation. Lower Singleton Formation Fine-grained volcanic Whole-rock geochemistry and Sm–Nd isotopes rocks in the lower Singleton Formation (Fig. 3) contain
Whilst every effort was taken to sample the freshest ma- 49 9–52 7 wt % SiO2, with high MgO (10 3–16 1 wt %), 2þ 2þ terial available, we nevertheless concentrate our discus- Mg-numbers [Mg /(Mg þFeTotal) x 100] (62–71), Ni sion on elements considered immobile (e.g. major (243–647 ppm) and Cr (810–2020 ppm) (Fig. 10). elements, high-field strength elements (HFSE), rare Samples with 18 wt % MgO contain abundant olivine 2144 Journal of Petrology, 2019, Vol. 60, No. 11 Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021
Fig. 6. Lordy Basalt Member photomicrographs. (a) PPL and (b) XPL images of zoned acicular pyroxene phenocrysts with augite rims and pigeonite cores (pseudomorphed by chlorite) in a fine, altered groundmass (sample 217771, 10 wt % MgO); (c) PPL and (d) XPL images of coarse acicular zoned pyroxene phenocrysts with augite rims and orthopyroxene cores (now altered to chlorite-ser- pentine) in a trachytic-textured groundmass of plagioclase and pyroxene microlites (sample 217796, 17 wt % MgO); (e) platy pyrox- ene spinifex textured flow top (sample 217703, no compositional data) and (f) XPL image of orthocumulate with relicts of olivine (mostly serpentinized) and poikilitic orthopyroxene and augite (sample 217714, 27 wt % MgO). phenocrysts and while these rocks are described else- relatively magnesian (6 1–12 6 wt % MgO) and under where as komatiitic (e.g. Barley et al., 2000), their MgO the IUGS classification (Le Maitre et al., 2002) most concentrations have clearly been inflated via olivine ac- samples classify as basalt, with lesser basaltic andesite cumulation. Based on fine grained sample composi- and picrite (48 8–52 8 wt % SiO2 and 1 6–3 5wt% Na2O tions and olivine fractionation trends, liquid þ K2O). They are relatively primitive, with moderate to compositions are estimated to be 16–20 wt % MgO and high Mg-numbers (48 to 68), Ni (93–462 ppm) and Cr hence picritic (Supplementary Data Electronic Appendix (284–1476 ppm), and have relatively low (0 57–0 76 wt 4a). However, the rocks have major element contents %) TiO2 concentrations. Although they do not meet the and trace element patterns similar to Al-depleted ko- major element criteria for boninite by either the IUGS matiitic basalts (e.g. Komati Formation, Barberton; (Le Bas, 2000; MgO > 8 wt %, SiO2 > 52 wt %, TiO2 < Robin-Popieul et al., 2012)(Figs 11a and d and 12a). In 0 5 wt %) or the more recent Pearce & Reagan (2019) the case of the komatiitic basalts, and probably the scheme (Fig. 11a and b;Si8 > 52, Ti8 < 0.5), most sam- Singleton Formation basalts and picrites, their unfrac- ples have LREE-depleted or concave mantle-normalized tionated to slightly depleted LREE patterns indicate a trace element patterns (Fig. 12b) and low Ti/V typically weakly depleted mantle source, while their high [Gd/ observed in Cenozoic boninite-related suites.
Yb]N (1 2–1 9; Fig. 11g) and low Al2O3/TiO2 (8–11 for Sub-volcanic rocks have SiO2 contents (48 8to lower Singleton Formation picrites; Fig. 11d) indicates 53 5 wt %) similar to volcanic rocks from the same local- retention of garnet as a residual phase (Arndt et al., ities, but reach higher MgO (7 3–19 1 wt %), Ni (105– 2008). 822 ppm), Cr (320–3170 ppm), and lower TiO2 (0 1– A single sample yielded eNd2820 of 1.2 (Table 2). 0 6 wt %) concentrations. Approximately 20% of sub- Upper Singleton Formation Fine-grained volcanic volcanic samples classify as boninite by the IUGS rocks from the upper Singleton Formation (Fig. 3) are scheme and 40% classify as boninite by the Pearce & Journal of Petrology, 2019, Vol. 60, No. 11 2145 Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021
Fig. 7. Variolitic-textured mafic volcanic rocks of the Meekatharra Formation: (a) varioles in siliceous high-Mg basalt (near sample 217771); (b) varioles coalescing in hand sample 221664; (c) varioles in thin section (PPL) of vitrophyric boninite-like rock (sample 221630); several varioles highlighted by white dashed outlines; (d) large variole in thin section (PPL) of high-Mg andesite west of Cue (sample 217784).
Reagan (2019) scheme, although compositions with < Six samples yield a narrow range in eNd2820 from 0 2 wt % TiO2 correspond to samples containing abun- þ0 2toþ0 9* (Table 2; *excluding altered cumulate 147 144 dant stubby to acicular pyroxene pseudomorphs (6 Cr- sample 221754 with eNd2820 þ2 0, since Sm/ Nd is spinel) with little groundmass (i.e. pyroxene orthocu- assumed to have been modified by metasomatism). mulates) and are, therefore, subject to cumulate effects.
Sub-volcanic units containing > 0 2 wt % TiO2 are most- Yaloginda Formation ly sub-ophitic with similar incompatible trace element The Yaloginda Formation (Figs 1 and 2) can also be patterns to the volcanic units (at slightly lower concen- divided into two distinct chemical groups. First, the trations; Fig. 12b). ‘high-HFSE series’ in the lower parts of the Yaloginda Pyroxene orthocumulate samples have very low in- Formation comprises basaltic andesites, dacites and compatible trace element concentrations, with patterns rhyolites (Quinns Basalt and Rhyolite, and Kantie that are either strongly LREE-MREE depleted (e.g. Murdana Volcanics; Figs 1 and 2) with high HFSE-REE 221755 in Fig. 12b) or LREE-enriched and MREE concentrations and relatively flat mantle-normalized depleted (i.e. concave; e.g. 221758, 221759 in Fig. 12b), trace element patterns. They are spatially and tempor- these patterns resemble boninites from the Troodos ally associated with layered intrusive complexes of Ophiolite in Cyprus (e.g. Osozawa et al., 2012) and the Meeline Suite. One representative trace element Ogasawara Islands (previously Bonin Islands; e.g. pattern is shown in Fig. 12c; this unit will be dis- Kanayama et al., 2012). Two cumulate samples (221754 cussed in greater detail in a subsequent study of felsic and 221756* [*not plotted]) have trace element patterns lithologies. Sm–Nd isotope data for sample 193923 in- similar to some Al-enriched komatiites (e.g. dicate an eNd2815 of 0 0 (WACHEM database, GSWA, Weltevreden Formation and Commondale komatiites; 2018 b). Wilson, 2003a, 2003b; Puchtel et al., 2013), however, The second chemical group in the Yaloginda they appear to have suffered post-volcanic metasoma- Formation is a series of high-Mg basaltic andesite, an- tism based on minor LREE-loss (relative to Zr–Hf, i.e. desite, dacites and rhyolite (hence ‘BADR-series’). 0 5 0 5 high Zr/Zr*[where Zr* ¼ NdPM SmPM ]), LILE gains Samples from stratigraphically lower levels contain (very high Cs, Ba, Rb; K2O1 4 wt % in 221754) and lower 52 9–55 1 wt % SiO2 with high MgO (8 9–11 7 wt %), Mg- MgO/TiO2 compared to other samples in this group numbers (57–68), Ni (246–441 ppm) and Cr (797– (Figs 10d and 12b), and are, therefore, not considered 2092 ppm), most samples are consistent with a high-Mg further in the discussion below. (or primitive) andesite classification, i.e. SiO2 > 54 wt % 2146 Journal of Petrology, 2019, Vol. 60, No. 11 Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021
Fig. 8. Bassetts Volcanic Member photomicrographs with PPL images on left and XPL images on right. (a) and (b) fine acicular pyr- oxene microphenocrysts with augite rims and pigeonite cores and euhedral orthopyroxene phenocrysts set in a glassy ground- mass (sample 221621, 13 wt % MgO); (c) and (d) acicular zoned pyroxene phenocrysts with augite rims and pigeonite cores (variably altered) within a felted groundmass of plagioclase and pyroxene microlites (sample 221693, 10 wt % MgO) ; (e) and (f) aci- cular zoned pyroxene phenocrysts with augite rims and pigeonite cores within a groundmass of dendritic pyroxene and glass (sam- ple 221699, 12 wt % MgO); (g) and (h) orthopyroxene cumulate with interstices filled by microlitic plagioclase, augite and opaque minerals (sample 81515, 20 6 wt % MgO). and MgO > 6wt % (Wood & Turner, 2009)andMg- is a sequence of andesitic to dacitic rocks with number 50–60 (> 60 for primitive andesite; Kelemen moderate concentrations of MgO (6 5–1 4wt%),Fe2O3 et al., 2007). They have mantle-normalized trace elem- (8 1–3 9wt%),TiO2 (0 9–0 6 wt %), Ni (150–10 ppm) and ent patterns that are LREE-enriched ([La/Sm]PM 2 3, Cr (296 to < 10 ppm), which decrease with increasing [Gd/Yb]PM 1 3) and have prominent negative Nb- SiO2 (56 4–63 4 wt %) as is expected for a transition anomalies relative to Th and La (Fig. 12c). These char- from crystallization of mafic to felsic mineral acteristics are comparable to Cenozoic high-Mg ande- assemblages. sites from volcanic arc settings, such as the Setouchi Sm–Nd isotope data for two samples yield eNd2815 - Volcanic Belt in SW Japan (e.g. Tatsumi et al., 2006). 0 1andþ0 2(Table 2), while another yields eNd2815 þ1 5 Overlying the Yaloginda Formation high-Mg andesites (sample 185930; WACHEM database, GSWA, 2018b). Journal of Petrology, 2019, Vol. 60, No. 11 2147 Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021
Fig. 9. Meekatharra Formation Tholeiitic basalts with PPL images on left and XPL images on right: (a) and (b) Stockyard Basalt Member (sample 217790); (c) and (d) Cue basalt (sample 217800).
Meekatharra Formation Sm–Nd isotopic data for two samples yield eNd2800 of The Meekatharra Formation (Figs 1 and 2) can be div- -0 2 and þ0 1(Table 2). ided into four distinct chemical members, the Lordy Bassetts Volcanic Member Fine-grained samples Basalt Member, Bassetts Volcanic Member, Stockyard from this unit yield high SiO2 (51 8–56 7 wt %), MgO Basalt Member and Bundle Volcanic Member: (8 1–17 2 wt %), and Mg-numbers (63–77). Samples with Lordy Basalt Member Fine-grained volcanic rocks > 15 wt % MgO contain abundant orthopyroxene phe- contain 49 2–54 6 wt % SiO2,6 1–20 4 wt % MgO, and nocrysts and those with 19–21 wt % MgO are orthopyr- their Mg-numbers range from 53 to 76. Samples con- oxene orthocumulates, implying liquid compositions taining > 16 wt % MgO are rare, and these contain had Շ 15 wt % MgO. Ni (109–513) and Cr (203– abundant fine-grained (< 1 mm), mostly euhedral oliv- 1980ppm) concentrations again vary significantly, but ine crystals. Hence, liquid compositions were almost correlate with MgO, reflecting olivine and orthopyrox- certainly Շ 16 wt % MgO. Ni (68–861 ppm) and Cr (87– ene control. TiO2 is low (0 39–0 60 wt %) and Al2O3 is 2757 ppm) concentrations in the Lordy Basalt Member relatively high (10 0–15 2 wt %), resulting in consistently vary significantly. TiO2 (0 53–0 86 wt %) is relatively low super-chondritic Al2O3/TiO2 ratios (24 4–27 5) that clear- (but not as low as to classify as boninite, < 0 5 wt %), ly differentiate the Bassetts Volcanic Member from the and Al2O3 (9 2–15 wt %) is moderate, resulting in rela- rest of the Meekatharra Formation samples (Al2O3/TiO2 tively constant, slightly sub-chondritic Al2O3/TiO2 (15– 10–20). Most samples classify as boninite in the IUGS 18). According to the IUGS classification scheme (Le scheme (Le Bas, 2000), but rare examples containing < Bas, 2000), various samples in this unit classify as pic- 10 wt % MgO exceed the upper 0 5 wt % TiO2 limit and rite, basalt or basaltic andesite, whereas in the Pearce & are instead classified as basaltic andesite (Fig. 11b). Reagan (2019) scheme, most samples classify as sili- According to the Pearce & Reagan (2019) scheme, most ceous high-Mg basalt (Fig. 11a and b). samples classify as siliceous high-Mg basalts, with only Trace element patterns are transitional between the lowest TiO2 samples classifying as boninite. CaO those of the overlying Bassetts Volcanic Member and ranges from 7 5–11 1 wt % and CaO/Al2O3 (0 65–0 82) is Bundle Volcanic Member (Fig. 12d), with fractionated in the range of high-Ca boninite (CaO/Al2O3 > 0 7–1;
REE patterns ([La/Nd]PM 1 4–1 5, [La/Yb]PM 1 4–2 0), Crawford et al., 1989). strongly negative Nb anomalies ([Th/Nb]PM 4 2–4 5 and Mantle-normalized trace element patterns have sub- [Nb/La]PM 0 4–0 5) and moderately positive Zr–Hf tly concave shapes, with minima near Nd (Fig. 12e), like anomalies (Zr*/Zr ¼ 1 15–1 30). In these very well- most Archean boninite-like rocks (e.g. Boily & Dion, preserved samples, Zr–Hf anomalies are considered to 2002; Manikyamba et al., 2005; Angerer et al., 2013). be an igneous feature. REE and HFSE ratios remain rela- They feature strong negative Nb anomalies [Th/Nb]PM tively constant over a significant fractional crystalliza- (4 8–5 7) and positive Zr–Hf anomalies (Zr*/Zr ¼ 1 3– tion interval (20 4–9 1 wt % MgO), indicating liquid 1 4). [Nb/Yb]PM (0 42–0 48) and [Gd/Yb]PM (0 92–0 98) evolution controlled mainly by olivine, which has low ratios are lower than in modern N-MORB (0 53 and 1 0 partition coefficients for those incompatible elements. respectively; Sun & McDonough, 1989), indicating a 2148 Journal of Petrology, 2019, Vol. 60, No. 11 Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021
Fig. 10. Major and trace element variation diagrams for Norie and Polelle Group samples: (a) SiO2 vs MgO; (b) Fe2O3 (total Fe as 3þ Fe ) vs MgO; (c) Al2O3 vs MgO; (d) TiO2 vs MgO; (e) Cr vs MgO; (f) [La/Nd]PM vs MgO; (g) [Gd/Yb]PM vs MgO; Th/Nb vs MgO. Journal of Petrology, 2019, Vol. 60, No. 11 2149 Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021
Fig. 11. Major and trace element variation and discrimination diagrams comparing Norie and Polelle Group mafic volcanics (includ- ing olivine-rich cumulates) to potential analogues: (a) MgO vs SiO2 and (b) MgO vs TiO2 discrimination diagrams (both after Pearce & Reagan, 2019; SHMB, siliceous high-Mg basalt); (c) MgO vs Al2O3; (d) Al2O3 vs TiO2; (e) Yb vs MgO; and (f) Zr/Zr* vs MgO. Data sources: Ogasawara HSB (High-Si boninite: Kanayama et al., 2012), Troodos LSB (low-Si boninite: Ko¨ nig et al., 2008; Osozawa et al., 2012; Golowin et al., 2017), Opatica (Boily & Dion, 2002), Whundo (Smithies et al., 2005), Dharwar (Manikyamba et al., 2005), Weltevreden Formation AEK and Komati Formation ADK (Al-enriched komatiite, Al-depleted komatiite; Robin-Popieul et al., 2012), Commondale AEK (non-cumulates only; Wilson, 2003 b; Hoffmann & Wilson, 2017), Belingwe AUK (Al-undepleted komatiite; Puchtel et al., 2009). 2150
Table 3: Chemostratigraphic characteristics of Norie and Polelle Groups
Stratigraphic Member Age (Ma) Type Locality IUGS classifica- SiO2 Mafic charac- REE patterns eNdT Melt fraction Source region Intrusive Tectonic Group and tion (chemical ter-istics (simple batch affinities implications Formation description) melt model)
Polelle Group: upper c. 2750 to Wattagee Hill Andesite to rhyo- High Mostly low Strongly enriched 2 2toþ0 5 not modelled Metasomatised Cullculli Suite Subduction Greensleeves (informal) 2740 lite (calc-alka- Mg, low Ni, LREEs, depleted to (n¼2) mantle; locally (horn- and/or Formation line, BADR- Cr flat HREEs, occa- contaminated blende-bio- underplat- series) sionally concave by old crust tite sanuki- ing of meta- between Dy-Yb toids and somatized Polelle Group: Woolgra c. 2760 to Polelle Basaltic andesite High High Mg, Ni, Strongly enriched þ0 8toþ1 not modelled Metasomatised TTGs) mantle Greensleeves Andesite 2740 Syncline to rhyolite Cr LREEs, depleted to (n¼2) mantle Formation (calc-alkaline, flat HREEs, occa- BADR-series) sionally concave between Dy-Yb Polelle Group: Cue basalt <2800 Cue Hill Basalt (siliceous High Moderate Mg, Strongly depleted – c. 5% 2nd Metasomatized Boodanoo Subduction Meekatharra (informal) high-Mg basalt) High Fe, LREEs through to stage melt refractory and and/or Formation low Ni, Cr HREEs of 10% mantle Warriedar underplat- depleted Suites ing of meta- DMM (hornblende somatized Polelle Group: Bundle <2800 Polelle Basaltic andesite High High-Mg, Ni, Strongly enriched þ0 2(n¼1) not modelled Metasomatised bearing mantle Meekatharra Volcanics Syncline to andesite (sili- Cr LREE through to mantle; pos- gabbros) Formation ceous high-Mg gently sloping sible melting basalt) HREEs (possible depth of 3 garnet signature) GPa Polelle Group: Stock-yard <2800 Polelle Basalt to andesite Nor-mal to Low to moder-Depleted to flat LREEsþ0 80 to þ1 2 not modelled Weak to moder- None Extension/ Meekatharra Basalt Syncline (tholeiitic High ate Mg, low and typically flat (n¼2) ately depleted identified rifting Formation basalt) Ni, Cr HREEs, but occa- un-metasoma- sionally depleted tised mantle; HREEs (garnet replenished up- signature) welling asthenosphere Polelle Group: Bassetts <2800 Polelle Basaltic andesite High High Mg, Ni, Concave REEs, 0 5(n¼2) c.20-25% 2nd Metasomatised Boodanoo Subduction Meekatharra Volcanics Syncline to andesite Cr strongly enriched stage melt refractory and and/or Formation (boninite-like si- La–Sm, [Gd/Yb]PM of 10% mantle Warriedar underplat- liceous high- 0.9. DMM Suites ing of meta- Mg basalt) (hornblende somatized
Polelle Group: Lordy Basalt <2800 Polelle Basalt to andesite High High Mg, Ni, Moderately enriched 0 2toþ0 1 not modelled Metasomatised bearing mantle Petrology of Journal Meekatharra Member Syncline (siliceous high- Cr LREEs, flat HREEs (n¼2) mantle melts or gabbros) Formation Mg basalt) contamination of fertile as- thenospheric melts Norie Group: Undiffer-enti- 2805 to Polelle Basaltic andesite High High Mg, Ni, Strongly enriched 0 1toþ1 4 not modelled Metasomatised Mount Yaloginda ated (BADR 2815 Syncline to rhyolite (sili- Cr LREEs, [Gd/Yb]PM (n¼4) lithosphere Kenneth Formation series) ceous high-Mg Suite (felsic basalt to high- intrusions Mg andesite) with calc-al- 11 No. 60, Vol. 2019, , kaline affinity)
(continued) Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021 September 27 on guest by https://academic.oup.com/petrology/article/60/11/2131/5700738 from Downloaded ora fPetrology of Journal 09 o.6,N.11 No. 60, Vol. 2019, , Table 3. Continued
Stratigraphic Member Age (Ma) Type Locality IUGS classifica- SiO2 Mafic charac- REE patterns eNdT Melt fraction Source region Intrusive Tectonic Group and tion (chemical ter-istics (simple batch affinities implications Formation description) melt model)
Norie Group: Kantie c. 2814 to Meeline Suite Rhyolite (locally High Low Mg, Ni, Weakly enriched 0(n¼1) not modelled Fractionated tho- Mount Local expres- Yaloginda Murdana, 2817 Intrusions andesitic) (tho- Cr LREEs, flat HREEs, leiitic liquids Kenneth sions of ex- Formation Youan- leiitic rhyolite) very high derived from Suite (felsic tension/ garra, concentrations melts of crust/ intrusions rifting Yuinmery lithospheric with tholei- Volcanics mantle itic affinity) Norie Group: Quinns Basalt >2817 Quinns Basalt to andesite High Low Mg, Ni, Weakly enriched – not modelled Singleton Mining (tholeiitic bas- Cr LREEs, flat HREEs, Formation District altic andesite) high concentrations Norie Group: upper >2817 Gaban-intha Basalt to basaltic High High Mg, Ni, Depleted to enriched þ0 9toþ0 2 c.20-25% melt Metasomatised Meeline Suite Subduction Singleton (informal) andesite Cr LREEs, [Gd/Yb]PM (n¼5) of DMM mantle (vari- (tholeiitic and/or Formation (boninite-like 0.7 to 1.1, locally source ably depleted) intrusions) underplat- rocks and high- concave patterns ing of meta- Mg basaltic somatized andesites) mantle Norie Group: lower >2817 Gaban-intha Basalt, picrite (Al- Nor-mal High Mg, Ni, Flat LREEs, depleted þ1 2(n¼1) c.10% melt of low-moderate de- Extension, rift- Singleton (informal) depleted ko- Cr HREEs; locally con- DMM gree melting ing; pos- Formation matiite-like vex patterns source or (mostly Cpx) in sible mantle high-Mg basalt, c.25% melt fertile upwell- plume picrite and of weakly ing astheno- orthocumulate) depleted sphere; garnet mantle at left in source 3 GPa
2151 Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021 September 27 on guest by https://academic.oup.com/petrology/article/60/11/2131/5700738 from Downloaded 2152 Journal of Petrology, 2019, Vol. 60, No. 11 Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021
Fig. 12. Trace element patterns for northwest Youanmi Terrane normalized to primitive mantle (Sun & McDonough, 1989). Partially transparent patterns used for comparisons include Komati Formation Al-depleted komatiites (BD4 and BD7; Robin-Popieul et al., 2012), Weltevreden Formation Al-enriched komatiites (WP107 and WP108; Robin-Popieul et al., 2012), Commondale Al-enriched ko- matiite (27 37; Wilson, 2003 b), Ogasawara high-Si boninite (Kanayama et al., 2012), Troodos low-Si boninite (CY119; Osozawa et al., 2012), Setouchi HMA (OTO-7; Tatsumi et al., 2006), Belingwe ‘E-Basalt’ (BX229; Shimizu et al., 2005), Sanukitoid avg and < 3 0 Ga TTG (tonalite–trondhjemite–granodiorite) avg (Martin et al., 2005). moderately depleted mantle source. In contrast, REE including those from the upper Singleton Formation patterns are mildly LREE enriched ([La/Nd]PM (1 4–1 6) (this study). If these signatures are indicative of their pri- and [La/Yb]PM (1 0–1 .3)). mary melts, then they suggest a less refractory mantle REE concentrations and [Gd/Yb]PM ratios in the source than for ‘typical’ boninite. As is the case Bassetts Volcanic Member are higher than in most throughout the Meekatharra Formation, REE and Cenozoic boninites and Archean boninite-like rocks, HFSE ratios show little variation as MgO decreases Journal of Petrology, 2019, Vol. 60, No. 11 2153
(17 2–9 0 wt %), confirming that olivine and orthopyrox- enriched mantle source or interaction with continental ene dominated the crystalizing mineral assemblages. crust, and [Nb/Yb]PM ratios greater than primitive man- Sm–Nd isotopic data for two samples yield identical tle values (1 2–1 6), again indicating an enriched mantle eNd2800 of -0 5(Table 2). source, or possibly retention of garnet in the mantle Stockyard Basalt Member The Stockyard Basalt source.
Member is chemically distinct from other volcanic units A single sample from this unit yielded eNd2800 þ0 2 in the Meekatharra Formation. Sample compositions (Table 2) vary between low- to moderate-Mg basalt and basaltic andesite (90% of samples contain 49 6–56 2 wt % SiO2 Greensleeves Formation and 4 1–8 0 wt % MgO). Based primarily on incompat- The Greensleeves Formation (Figs 1 and 2) consists of ible element concentrations, the unit can be separated an inferred differentiation series of basaltic andesite-
into two sub-populations. The first population (popula- andesite-dacite-rhyolite (BADR). The analysed samples Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021 tion 1: Murrouli Range, Fig. 2) is slightly more magnes- map this trend, from 55 2to73 4 wt % SiO2 and 7 3to ian than the second (population 2; Polelle Syncline and 0 01 wt % MgO (> 1 3% in samples with < 70 wt % Murrouli Range, Fig. 2), and contains significantly lower SiO2). Mg-numbers vary from 66 to 45 (but mostly > 50) TiO2 (0 7–1 0 wt % vs 1 3–1 7 wt %), HFSE (especially Nb in basaltic andesite to dacite compositions. Compared and Zr) and REE concentrations than population 2. to other Archean felsic rocks, including TTGs, Despite large concentration differences for incompat- Greensleeves basaltic andesite–dacitic compositions ible elements between the two populations, mantle- are relatively high in Ni (49–272 ppm for andesite, 25– normalized trace element patterns are broadly similar 106 ppm for dacite) and Cr (78–713 ppm for andesite (Fig. 12f); most are Th- and Nb-depleted ([Th/Yb]PM 0 6– and 34–204 ppm for dacite). Trace element patterns are
1 0, [Nb/Yb]PM 0 5–1 1) and LREE depleted relative to strongly LREE-enriched ([La/Yb]PM > 13 6 5). Compared MREE–HREE, which are relatively unfractionated. to ‘typical’ Archean TTGs (e.g. Martin et al., 2005), Exceptions to that more common trace element pattern Greensleeves Formation samples are more magnesian, include several samples from population 1 with convex- contain higher Ni and Cr, and have less fractionated down (hump-shaped) patterns ([La/Sm]PM 0 51–1 11 trace element patterns, in these compositional charac- and [Gd/Yb]PM 0 96–1 47), possibly indicating retention teristics they are transitional between high-Si adakites of garnet in their mantle source, and five samples from (HSA; Martin et al., 2005), and Cenozoic HMA (Tatsumi population 2 that are Th-enriched with strong negative et al., 2006). Nb anomalies ([Th/Nb]PM 1 4–2 0), undepleted LREE, weakly depleted HREE ([La/Sm] 0 93–1 12 and [Gd/ PM DISCUSSION Yb]PM 1 06–1 30), and small positive Zr–Hf anomalies (Zr/Zr* 1–1 2). Comparison of Archean boninite-like rocks with Two samples yielded slightly positive eNd2800 þ0 8 Cenozoic boninites and þ1 2(Table 2). ‘Boninites’ are defined by the IUGS as volcanic rocks
Bundle Volcanic Member Fine-grained samples from with SiO2 > 52 wt %, MgO > 8 wt % and TiO2 < 0 5wt% the Bundle Volcanic Member have high, but variable (Le Bas, 2000). A more recent classification scheme
SiO2 (50 9–62 3 wt %), MgO (5 7–16 6 wt %) and Mg- (Pearce & Reagan, 2019) has suggested including rocks numbers (55–74). Ni (96–566 ppm) and Cr (267– with lower SiO2 concentrations in the boninite classifi- 1470 ppm) concentrations vary significantly but correl- cation if their fractionation trends project backwards to ate with MgO. TiO2 is moderately high (0 6–1 1 wt %), SiO2 52 wt % and TiO2 0 5 wt % at 8 wt % MgO (Si8, and Al2O3 (9 0–13 6 wt %) is lower than other high-Mg Ti8) which includes boninitic rocks with a significant mafic volcanic rocks in the Meekatharra Formation (at olivine phenocryst component. Rocks meeting that def- equivalent MgO concentrations), resulting in lower inition can vary significantly in terms of their incompat-
Al2O3/TiO2 (11–18). Their compositions broadly overlap ible trace element characteristics, which are thought to with the Lordy Basalt Member, but can be distinguished reflect variable degrees of source depletion and condi- by their slightly lower Al2O3 and CaO, and higher SiO2 tions of melting (Crawford et al., 1989). For example, concentrations. boninites from the Ogasawara Islands contain high
Bundle Volcanic Member samples have very uniform MgO (8–15 wt %) and SiO2 (57–60 wt %) and very low mantle-normalized trace element patterns that are typ- TiO2 (0 1–0 2 wt %) and HREE concentrations, indicating ically more LREE-enriched ([La/Yb]PM 3 1vs 1 6 and a near-clinopyroxene-free source with melting involv- [La/Sm]PM 1 5–2 5vs1 3–2 0), and have higher [Gd/ ing mostly orthopyroxene. Boninites from the Troodos Yb]PM (1 4–1 5vs1 0–1 2) than the Lordy Basalt Ophiolite in Cyprus, on the other hand, have lower MgO Member, and they also lack the Zr–Hf anomalies ( 10 wt %) and SiO2 (53–56 wt %), but higher TiO2 (0 2– observed in the other high-Mg mafic volcanic rocks of 0 4 wt %) and HREE concentrations, indicating a the Meekatharra Formation (Zr/Zr* ¼ 1 6 0 1) (Figs 11f depleted lherzolite source and melting that included and 12g). They have strong negative Nb anomalies ([Th/ clinopyroxene. These two type examples are some-
Nb]PM 4 4–4 9 and [Nb/La] N 0 4–0 5), indicating an times described as high-Si and low-Si boninites, 2154 Journal of Petrology, 2019, Vol. 60, No. 11 respectively (Kanayama et al., 2012; Umino et al., 2017; with intense mantle depletion at shallow depths in a Pearce & Reagan 2019). proto-arc wedge or a back-arc environment (i.e. less Many Archean ‘boninite-like’ rocks display high-Mg than c.2 GPa or c.60 km: Stern & Bloomer, 1992; Hickey- ( 8 wt %), low-Ti ( 0 5 wt %) and strongly depleted in- Vargas et al., 2018; Woelki et al., 2019). In at least some compatible trace element characteristics similar to cases, however, the initial mantle depletion associated
Cenozoic boninites, but contain lower SiO2 (< 52 wt %) with Archean LOTI may have been produced or (Kerrich et al., 1998; Wyman et al., 1999; Polat et al., enhanced by high temperatures associated with a man- 2002, Manikyamba et al., 2005). By analogy with post- tle plume (Wyman, 2003; Smithies et al., 2004), similar Archean counterparts (Brown & Jenner, 1989, Pearce & to a scenario invoked for Tongan boninites by Reagan, 2019), such rocks are sometimes described as Danyushevsky et al. (1995). ‘low-Ti tholeiites’ (LOTI; Kerrich et al., 1998; Wyman Another class of hypotheses applied to some
et al., 1999) but were termed ‘Whitney-type’ boninites Archean boninite-like rocks is that they are the contam- Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021 by Smithies et al. (2004). ination products of other mantle magma types. The Some Archean suites do in fact meet the chemical suggestion that siliceous high-Mg basalts may be con- definition of boninite s.s. (Fig. 11a, b and e), including taminated examples of Archean boninitic magmas (Sun examples from the 2 79 Ga Opatica Sub-Province et al., 1989) is rarely cited in the recent literature, but nu- (Superior Province, Canada; Boily & Dion, 2002), merous studies have proposed a contaminated komati- c.2 7 Ga Gadwal greenstone belt (Dharwar Craton, ite model for komatiitic basalts or siliceous high-Mg India; Manikyamba et al., 2005), and 3 12 Ga Whundo basalts (e.g. Arndt & Jenner, 1986; Arndt et al., 2001; Group of the West Pilbara Terrane (Pilbara Craton, Shimizu et al., 2005; Barnes et al., 2012), which, like Western Australia; Smithies et al., 2005). These exam- boninites, have high Si and Mg, and low Ti, as well as ples have MgO vs TiO2 and HREE characteristics of low- strongly enriched LREE. Shimizu et al. (2005) reported Si boninites (as defined by Pearce & Reagan, 2019: several siliceous high-Mg basalts (e.g. BX235, BX229) Fig. 11a, b and e). However, these rocks generally differ from the Belingwe greenstone belt that have strikingly from Archean LOTI basalts in having, on average, similar characteristics to boninite-like rocks in the slightly higher SiO2 (c.52–54 wt % at 8 wt % MgO; at the Meekatharra Formation, including concave REE pat- lower end of the Cenozoic boninite range) and concave terns (Fig. 12e), high SiO2 (c.54 wt % at 9 wt % MgO) and REE patterns, indicating a second, LREE-enriched high Al2O3/TiO2 ratios (c.25). The time interval between source component. As is the case for some Archean the siliceous high-Mg basalt-hosting Zeederbergs LOTI basalts (e.g. Wyman et al., 1999), these examples Formation and the underlying komatiite- and komatiitic occur within stratigraphic sequences containing other basalt-hosting Reliance Formation is poorly con- subduction-associated lithologies such as calc-alkaline strained, falling between a few million years and up to intermediate to felsic volcanic rocks (including adakites) c.70 Myr (Prendergast & Wingate, 2013). Shimizu et al. and intrusions (such as sanukitoids) that have been suggested that komatiitic magmas ponded in the crust, taken as evidence for subduction or subduction-like where they were contaminated and underwent fraction- processes (Boily & Dion, 2002; Smithies et al., 2005). al crystallization to generate the boninite-like character- A comparison between the chemical attributes of istics of these lavas. However, it is difficult to conceive Cenozoic boninites, and those of Archean LOTI and of an assimilant mixture that would result in both rela- boninite-like rocks, is presented in Table 4. tive depletion of the MREE and enrichment of the LREE, Similarities between boninites and the Archean Al- while also generating positive Zr/MREE anomalies that enriched Commondale suite have long been noted are not observed in the associated Al-undepleted (Wilson, 2003a, 2003b). Under the new Pearce & komatiites. Moreover, Reliance Formation komatiites Reagan (2019) classification scheme, the suite is now that host xenolithic garnets and clinopyroxenes, or their classed as ‘komatiitic boninite’, based on their combin- depleted basalt fractionation products, do not display ation of boninite compositions and spinifex textures, al- boninite-like REE patterns or the high Th/LREE found in though those textures are defined by orthopyroxene the siliceous high-Mg basalts (Shimizu et al., 2004, spinifex rather than olivine spinifex (Barr et al., 2009). 2005). Various studies of the Al-enriched Commondale ultra- mafic suite have preferred either mantle plume (e.g. Robin-Popieul et al., 2012) or supra-subduction zone Classification and source characteristics of settings for melting (e.g. Wilson et al., 2003a; Hoffmann northwestern Youanmi Terrane mafic–ultramafic & Wilson, 2017), but there is general agreement that the volcanic rocks sources of the Al-enriched magmas must have yielded The following discussion focusses on high-Mg mafic earlier Al-depleted, high [Gd/Yb]N komatiitic melts at volcanic units within the Norie and Polelle Groups that high pressure, where majorite garnet is stable, before potentially preserve chemical attributes (e.g. trace yielding Al-enriched, low [Gd/Yb]N melts at lower pres- element ratios) of their mantle sources. Table 3 summa- sures, i.e. melting a source with a strong majorite gar- rizes the chemical attributes and petrogenetic interpre- net signature. In contrast, Phanerozoic boninites and tations of each chemostratigraphic unit defined based LOTI are commonly envisioned to be the associated on the results above. ora fPetrology of Journal
Table 4: Comparison between modern boninites and Archean LOTI and boninite-like suites
Geologic Cenozoic Cenozoic low- Archean LOTI Archean LOTI komatiitic Archean Archean Archean Archean Archean Archean 11 No. 60, Vol. 2019, , period and high-Si Si boninite suite suite boninite Boninite-like Boninite-like Boninite-like Boninite-like Boninite-like Boninite-like chemical suite boninite suite suite suite suite suite suite
Locality Ogasawara Cyprus - Isua Abitibi Commondale Whundo Opatica Dharwar upper upper Bassetts Islands Troodos Singleton Singleton Volcanic upper pillow Formation Formation Member lavas (Norie Group): (Norie Group): (Polelle Subvolcanic/ volcanic/fine Group): fine cumulate grained grained
MgO wt% 9-19 6-13 7-16 9-13 30-42 8-10 6-13 12-24 13-19 8-12 8-17 SiO2 wt% 55-61 53-56 47-54 48-53 45-51 50-53 49-58 45-52 50-53 48-53 52-57 TiO2 wt% <0 1-0 20 2-0 40 2-0 40 3-0 5 0 10 30 4-0 50 2-0 40 1-0 40 5-0 70 4-0 6 Si8 wt% 57-61 53-56 49-54 48-54 54-57 51-53 53-58 49-54 53-55 50-54 52-58 Ti8 wt% 0 1-0 20 2-0 40 2-0 40 3-0 50 2-0 30 30 4-0 50 3-0 50 1-0 50 6-0 70 4-0 6 Mg-number 65-79 57-72 58-75 63-72 86-92 63-68 62-72 67-80 71-78 57-68 63-77 Al2O3/TiO2 63-137 36-62 44-93 38-65 64-84 55-58 33-38 30-50 25-140 20-25 24-27 [Th/Nb]PM 1 9-7 10 7-2 20 9-4 91 1-1 50 2-0 8 (Th 2 1-5 63 1-5 32 4-7 31 5-4 71 7-5 14 8-5 7 typically not reported) [La/Sm]PM 0 9-2 10 6-1 50 6-1 41 0-1 50 1-1 5 (two 1 4-2 41 6-2 11 5-2 61 1-2 51 1-2 21 4-1 6 outliers >5) [Gd/Yb]PM 0 6-0 90 4-0 90 3-0 60 4-0 60 2-0 40 7-0 80 7-0 90 6-0 70 4-1 00 7-1 10 9-1 REE pattern Concave Concave Flat LREEs, Flat LREEs, Flat LREEs, Concave Weakly con- Weakly con- LREE depleted LREE depleted Weakly con- shape Minima Minima at strongly strongly strongly Minima at cave cave to concave to LREE cave varies be- Pr LREE-MREE LREE-MREE LREE-MREE Tb Minima at Minima at enriched Minima at tween Sm depleted depleted depleted Sm Sm Sm to Tb Zr/Zr* 1-3 80 8-1 21 1-2 41 0-1 40 8-5 40 6-0 90 9-2 10 7-1 10 9-1 10 9-1 41 3-1 4 Reference Kanayama Osozawa Polat et al., Wyman et al., Wilson 2003b; Smithies Boily & Dion, Manikyamba This study This study This study et al., 2012 et al., 2012; 2002 1999 Barr et al., et al., 2004 2002 et al., 2005 Ko¨ nig et al., 2009 2008; Golowin et al., 2017
2155 Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021 September 27 on guest by https://academic.oup.com/petrology/article/60/11/2131/5700738 from Downloaded 2156 Journal of Petrology, 2019, Vol. 60, No. 11
Singleton Formation Al-depleted high-Mg basalts These rocks were largely ignored in earlier studies, but and olivine cumulates they represent a petrogenetically important chemical The stratigraphically lowest unit in the study area is the end-member unit, occurring close to the onset of the lower Singleton Formation, which is dominated by major 2820–2730 Ma magmatic episode that dominates high-Mg basalts, picrites and olivine orthocumulates. the northwestern Youanmi Terrane. These have been described in other studies as (Al- Approximately 20% of high-Mg samples (> 8wt % depleted) komatiites (Reudavey, 1990; Watkins & MgO) from the upper Singleton Formation meet the Hickman, 1990; Barley et al., 2000; Hallberg, 2000; Van major element classification criteria for boninite under Kranendonk et al., 2013), but all samples with > 16 wt % the IUGS scheme (Le Bas, 2000), and approximately MgO contain cumulus olivine (picrites and olivine 40% (all samples with > 14 wt % MgO) meet those of orthocumulates; 21–36 wt % MgO; Fig. 4c–f). Using the the more recent scheme by Pearce & Reagan (2019) methods of Robin-Popieul et al. (2012), we estimate that (Fig. 11a and b), which accounts for compositions modi- Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021 the more primitive liquids contained 16–20 wt % MgO fied by olivine and orthopyroxene fractionation. (Supplementary Data Electronic Appendix 4a), which is However, on the plot of MgO vs TiO2 (Fig. 11b), most marginal between komatiite (> 18 wt % MgO; Le Bas, samples define a fractionation trend with a strong clino- 2000) and picrite (12–18 wt % MgO; Le Bas, 2000), but pyroxene influence (towards lower MgO/TiO2 than notably lower than most well-known Al-depleted expected for olivine or orthopyroxene control), and ap- komatiites (e.g. 26 wt % MgO for the Komati Formation, proximately half of those samples that plot in the bonin- South Africa; Robin-Popieul et al., 2012). ite field contain cumulus clinopyroxene phenocrysts.
Higher concentrations of Al2O3 and TiO2 (at similar Since the compositions of pyroxene orthocumulate MgO) compared to most Al-depleted komatiites and samples are subject to cumulate effects, the most primi- basalts (e.g. Komati Formation) indicate that clinopyr- tive liquid composition was almost certainly less mag- oxene was the main mineral phase consumed during nesian, transitional between finer grained sub-volcanic melting, while less magnesian compositions (< 16 wt % rocks and primitive volcanic rocks (12–14 wt % MgO, MgO and Mg-numbers 62–71 in fine-grained rocks vs 0 3–0 55 wt % TiO2, 51–52 5 wt % SiO2) and marginal be- 22–33 wt % MgO and Mg-numbers 66–87 in spinifex tex- tween basalt and boninite. tured rocks of the Komati Formation) indicate a smaller Most samples have incompatible element patterns melt fraction than typically inferred for komatiite. with the MREE depleted relative to HREE ([Gd/Yb]PM Simple trace element modelling using Petromodeler 0 66–1 07) and low [Nb/Yb]PM (0 52–0 85) and TiO2, (Ersoy, 2013) indicates their REE patterns, with relative- requiring a moderately depleted mantle source, such as ly unfractionated LREE–MREE and high [Gd/Yb]PM ratios the one that generated the underlying basalts and pic- (avg 1 55 vs 1 4 in Komati Formation), requires melting rites. Simple trace element melt-modelling was able to at high pressures where garnet is stable ( 3 GPa, approximate the MREE–HREE patterns with a 20–25% assuming an un-metasomatized source). The main batch melt of a source similarly depleted to the characteristics of these trace element patterns can be modern N-MORB source (Workman & Hart, 2005) recreated via a range of melt-crystallization models and (Supplementary Data Electronic Appendix 4b). two scenarios are considered here (see Supplementary The variable LILE–LREE enrichment observed in the Data Electronic Appendix 4b). The first model is via 10% trace element patterns of the upper Singleton batch melting of depleted garnet lherzolite at 3 GPa Formation clearly reflects varying contributions of a using a depleted MORB mantle (DMM) source compos- crustal component (e.g. via assimilation, mantle meta- ition, mineral/melt partition coefficients and melt modes somatism, magma mixing) which is discussed in sub- of Salters & Stracke (2004), and 15% fractional crystal- sequent sections. Dividing the upper Singleton lization. The second model is via 25% batch or dynamic Formation into two populations at a [La/Gd]PM ratio of melting of a more fertile mantle (35% depleted mantle 1 25 distinguishes unenriched-mildly enriched patterns and 65% primitive mantle) at 9 GPa using mineral/melt (30% of samples) from moderate-strongly enriched partition coefficients and melt modes of Robin-Popieul patterns (70% of samples). The more LREE-enriched et al. (2012; derived in part from Borg & Draper, 2003) samples ([La/Gd]PM > 1 25) generally have slightly and 10% fractional crystallization. Both models require lower TiO2, CaO, V and Sc, and slightly higher SiO2, minor interaction with a crustal component to slightly Ni and Cr, which possibly reflects a greater degree enrich Th, La and Ce, either via crustal assimilation (r- partial melt (greater Opx contribution, relative to Cpx). factor ¼ 0 1; assimilating a Narryer Terrane 3 3–3 5Ga Together with higher concentrations of Ba and Th, granite composition), or mixing with metasomatised this may indicate variable metasomatism of the lithosphere (not modelled). Both scenarios are consist- source. Bizimis et al. (2000) studied the trace element ent with asthenospheric upwelling in a rift setting. contents of clinopyroxenes from supra-subduction zone peridotites and found that the melting rate of cli- Singleton Formation boninite-like rocks nopyroxenes during hydrous mantle melting is less The oldest suite of rocks with boninite-like characteris- than in dry melting, but that the rate of orthopyroxene tics occurs in the c.2815 Ma upper Singleton Formation. depletion is greater. Journal of Petrology, 2019, Vol. 60, No. 11 2157
Meekatharra Formation boninite-like rocks Meekatharra Formation depleted tholeiites The second suite of boninite-like rocks in the study A second suite of Meekatharra Formation basalts, near area, the c.2800 Ma Bassetts Volcanic Member of the the town of Cue (Fig. 2), have lower SiO2 (49 7–51 8wt Meekatharra Formation, occurs throughout the north- %), are less magnesian (mostly 5–7 wt % MgO, Mg- ern Youanmi Terrane and is well-exposed in the Polelle number 40–50) and contain higher TiO2 (mostly 0 4– Syncline (Fig. 2). The rocks of the Bassetts Volcanic 0 8 wt %) than boninites, yet they are relevant to this Member can be readily distinguished from other discussion because of their low [Gd/Yb]PM (mostly 0 7– mafic volcanic rocks in the study area by their low-TiO2 0 85), which suggests a moderately refractory mantle (< 0 5 wt %), above-chondritic Al2O3/TiO2 ratios (c.25), source, similar to the source inferred for boninite-like concave REE patterns, and negative Nb–Ta and positive rocks in the Bassetts Volcanic Member. Simple melt- Zr–Hf anomalies. Most samples meet the IUGS defin- modelling indicates their source was also 10% depleted ition of boninite (Le Bas, 2000), but samples containing relative to N-MORB-source mantle, but with lower Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021
< 10 wt % MgO typically exceed the upper TiO2 limit degrees of partial melting (5% batch melt, assuming an (0 5 wt %), and locally may drop below the lower SiO2 anhydrous unmetasomatised source) than for boninite- limit (52 wt %). In the newer Pearce & Reagan (2019) like rocks in the study area (Supplementary Data scheme, most Bassetts Volcanic Member lavas plot Electronic Appendix 4b). Their lower MgO, Mg-number above the upper TiO2 vs MgO boundary of the boninite and higher TiO2, compared to coeval boninite-like rocks field, within the picrite/komatiitic basalt/siliceous high- in the Bassetts Volcanic Member, can, therefore, be Mg basalt field (Fig. 11a and b). explained by lower degrees of melting and possibly Boninite-like rocks from the Bassetts Volcanic higher degrees of fractional crystallization. Trace elem-
Member have asymmetrically concave REE patterns, ent patterns for the Cue basalts have low [La/Yb]PM similar to many Phanerozoic low-Si boninite suites, (mostly < 0 6) and small Nb-depletions (Fig. 12f), which but at generally higher MREE–HREE contents precludes significant source enrichment or crustal con-
(Fig. 12e). On average, they also contain higher TiO2 tamination and, therefore, constitute our best constraint and are less MREE-depleted than Phanerozoic low-Si on the trace element ratios of the mantle source prior to boninites, indicating a less depleted mantle source. LILE–LREE enrichment. Simple melt-modelling indicates a mantle source that was 10% depleted relative to N-MORB-source mantle Mode of mantle–crust interaction and required higher degrees of partial melting (20– The following sections discuss two competing hypothe- 25% batch melt) than the earlier boninite-like rocks to ses for LILE–LREE enrichment in boninite-like rocks: match MREE-HREE concentrations in the more primi- melting of a mantle source metasomatised by subduc- tive samples (Supplementary Data Electronic tion or sagduction and crustal contamination of initially Appendix 4b). Trace element patterns of Bassetts unenriched mafic–ultramafic primary melts. Volcanic Member samples feature ubiquitous Zr–Hf anomalies, which are not observed in the earlier boninite-like rocks. In the case of these very well- Th/Yb–Nb/Yb systematics preserved samples, the features are considered to be In modern oceanic settings, negative Nb anomalies in of igneous origin. In all of the contamination scenarios mantle-normalized trace element patterns (i.e. high Th/ that we modelled, the Zr–Hf anomalies could not be Nb or low Nb/La) are widely attributed to the relative reproduced (Electronic Appendix 4 b). In Cenozoic immobility of Nb (compared to other incompatible litho- boninites, positive Zr–Hf anomalies (high Zr/Zr* and phile elements) in hydrous fluids to the extent that high 0 25 0 75 high Hf/Hf* [Hf* ¼NdPM SmPM ]) have typically been Th/Nb is commonly used as a proxy for subduction en- attributed to enrichment of sub-arc mantle by fluids richment (Pearce & Stern 2006; Pearce 2008). (Bizimis et al., 2000) or melts (e.g. Ko¨ nig et al., 2010; Many Archean greenstones have high Th/Nb, lead- Kanayama et al., 2012) released from hydrated ocean- ing to suggestions that modern-style plate tectonic ic crust during subduction. Bizimis et al. (2000) found processes were operating during the Archean (see dis- that enrichments in LREE-MREE and HFSE (Zr) were cussion in Smithies et al., 2018). However, high Th/Nb linked to continuous hydrous flux of the mantle in can also be produced (or enhanced) in mantle-derived models for subduction settings that initiated refertiliza- melts by assimilation of high Th/Nb continental crust, tion after 9% and 10% melting of a MORB source. or by mixing with high Th/Nb crustal melts. One important piece of evidence regarding the nature One method of evaluating Th/Nb systematics uses of the mantle at 2800 Ma, when the Bassetts Volcanic Pearce’s (2008) plot of Th/Yb vs Nb/Yb. Discrete trends Member erupted, is the presence of the hornblende- in this plot may be able to differentiate between two rich Narndee Igneous Complex (eNd2800 ¼ -0 7toþ main modes of mantle-crust interaction: hydrous 2 0) in the central part of the Youanmi Terrane. Ivanic subduction-linked mantle metasomatism versus crustal et al. (2015) showed, isotopically, that the parental assimilation. The clearly dominant Th/Yb-Nb/Yb trend magmas of the Narndee Igneous Complex had hy- in our study area is oriented parallel to the mantle array drous mantle sources. and almost entirely within the modern arc array (red 2158 Journal of Petrology, 2019, Vol. 60, No. 11 Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021
Fig. 13. Th/Yb vs Nb/Yb after (Pearce, 2008). (a) Norie Group; (b) Polelle Group. Samples containing > 68 wt % SiO2 have been excluded.
symbols in Fig. 13a and b). This trend originates at Th/ have mantle-like [Th/Nb]PM (mostly < 1 and < 1 2 re- Yb equal to, and Nb/Yb lower than, modern N-MORB spectively) and unfractionated or depleted LREE pat- and increases at relatively constant Th/Nb ratios to- terns ([La/Nd]PM 0 9–1 1 and 0 7–1 0 respectively). wards typical Archean continental crust values. In de- Internal Th/Yb–Nb/Yb variation within each volcanic tail, the trend comprises several discrete groups of unit may reflect near-surface contamination (e.g. two samples relating to geographically or temporally dis- outliers at higher Th/Yb for the lower Singleton crete segments of the greenstone package and, there- Formation and five outliers at higher Th/Yb for the fore, likely reflects variable source fertility, melting Stockyard Basalt Member), but the volcanic units them- regimes and crust-mantle interaction. Nearly all high- selves cannot be petrogenetically related to one an- Mg compositions fall on this trend (except those other by crustal contamination. For example, belonging to lower Singleton Formation) and very few compositions from the Quinns Basalt Member have compositions plot between this trend and the modern relatively high [Th/Nb]PM (1 7–2 2) and are mildly LREE- mantle array, suggesting that the high Th/Nb ratios of enriched ([La/Nd]PM 1 2–1 3), but their near chondritic these rocks were inherited from the mantle source, or at MREE–HREE ratios ([Gd/Yb]PM 1 0–1 2) are inconsistent least prior to the magmas ascending through the crust. with a crustal contamination origin from parental melts
The crustal contributions to the parent magmas of the to either the lower Singleton Formation ([Gd/Yb]PM 1 5– samples that define this trend are discussed in the fol- 1 9) or Stockyard Basalt Member ([Gd/Yb]PM 1 0–1 4). lowing sections (see ‘Sm-Nd isotopic variation’). Instead, they resemble Fe–Ti–P rich basalt-andesite ser- Three volcanic units plot as discrete clusters along a ies rocks or ‘Icelandites’ (e.g. the basaltic icelandites of smaller sub-vertical trend, separated from the main Th/ NE Iceland: Jo´ nasson, 2005; and Archean examples Yb–Nb/Yb trend: (1) In the Polelle Group, Stockyard from the Kidd Creek Volcanic Complex, Abitibi belt, Basalt Member compositions plot as two clusters be- Canada: Wyman et al., 1999). tween ‘N-MORB’ and ‘primitive mantle’ values (Fig. 13b); (2) In the Norie Group, lower Singleton Formation compositions straddle the upper limit of the Sm–Nd isotopic variations modern MORB-OIB array (Fig. 13a); and (3) Quinns The total Nd isotopic range for the samples reported
Basalt Member (high-HFSE basaltic andesite) composi- here is surprisingly small (eNdT from þ1 2to–0 5; tions straddle the lower limit of the modern arc arrays excluding sample 221754 with eNdT þ2, because LREE (Fig. 13a). Considering Th/Yb–Nb/Yb in isolation, these loss likely affected 147Sm/144Nd; Table 2), and all data clusters could be interpreted as a contamination vector points plot along a c.2 82 Ga isochron (not shown). (e.g. Pearce, 2008). However, most of the samples that Interpretation of these results must take into account define the trend have trace element compositions indi- the analytical uncertainty (c. 6 0 5 units for eNdT). cating low-degrees of crustal interaction: the lower However, the observed isotopic range for the volcanic Singleton Formation and Stockyard Basalt Member rocks is very similar to that previously reported for the Journal of Petrology, 2019, Vol. 60, No. 11 2159 Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021
Fig. 14. Whole-rock Sm–Nd isotope systematics; eNdT vs magmatic age. Diagonal grey dashed lines indicate crustal isotope evolu- tion pathways assuming 147Sm/144Nd ¼ 0 115. Additional data sources: Southwest Youanmi Terrane data Windimurra and Narndee (Ivanic et al., 2015); Yaloginda Formation, Greensleeves Formation and Cullculli Suite (WACHEM database; GSWA, 2018 b); southwest Youanmi Terrane (unpublished data); Western Youanmi Terrane granites and gneisses (Champion, 2013 and references within); Narryer Terrane granites and gneisses (De Laeter et al., 1985; Nutman et al., 1993). broadly coeval mafic–ultramafic complexes in the re- lithosphere, or slab melts 6 sediment component) that gion, such as the c.2813 Ma Windimurra Igneous had much lower eNdT. Complex (þ1 8to-1 0) and the c.2800 Ma Narndee Published Sm–Nd isotope data for the northwestern Igneous Complex (þ2 0to-0 7; Ivanic et al., 2015). Yilgarn region indicate that eNd2800 ranged from þ4 Moreover, subtle isotopic differences between the (c.2 8 Ga olivine cumulates, Wyman & Kerrich, 2012)to Norie Group (eNdT þ1 5to-0 1; including data from -11.5 (3.6 Ga Meeberrie Gneiss, Narryer Terrane, Geological Survey of Western Australia, 2018b) and Nutman et al., 1993)(Fig. 14). Norie Group volcanic
Polelle Group (eNdT þ1 2to-2 2; including data from rocks show little variation in their Sm–Nd isotopic com- Geological Survey of Western Australia, 2018b) correl- positions, and nearly all samples have juvenile eNdT ate with differences in Th/Yb–Th/Nb systematics (þ1 5to-0 1), consistent with similar aged, juvenile gab- (Fig. 13), suggesting greater crustal influence in the bros of the Windimurra Igneous Complex in the central Polelle Group. The consistency of the results, their simi- Youanmi Terrane (Nebel et al., 2013), indicating little larity with the data for coeval intrusive mafic com- interaction with ancient continental crust from c.2820– plexes, and correlations with chemical features, 2800 Ma. In contrast, Polelle Group volcanic rocks and suggest that primary Sm–Nd isotope systems remained coeval intrusions (c.2799–2735 Ma) show greater isotop- largely undisturbed and calculated eNdT values repre- ic variation (eNdT þ1 2to-3 2). Nearly half have mildly to sent primary magmatic signatures. moderately evolved eNdT, including three high-Mg mafic Nd isotopic compositions of the Neoarchean mantle volcanic rocks (eNdT -0 2to-0 5) and four rhyolitic and are difficult to ascertain due to alteration effects and granitic rocks (eNdT –2 2to-3 2), indicating mixing or crustal assimilation (e.g. Lahaye et al., 1995; Arndt et al., contamination with an ancient continental crust compo- 2001; Frei et al., 2002; Said & Kerrich, 2010). Global nent (Figs 14 and 15). Lu–Hf isotopic compositions from models for the evolution of the depleted mantle (e.g. the western Youanmi Terrane (zircon Lu-Hf; Ivanic et al., DePaolo, 1981; Goldstein et al., 1984) are a guide and 2012) show similar overall trends: isotopic ratios be- suggest eNd2800 values between þ3 8 and þ2 1(Fig. 14). tween 2810–2760 Ma are mostly juvenile (eHfT þ3 6to- Nd isotope compositions in this range exist in some ju- 0 5; 95% > eHfT 0) and become more variable and venile Neoarchean granite–greenstone terranes (e.g. evolved from 2760 to 2735 Ma (eHfT þ2 4 to -10; 33% > Abitibi-Wawa, Canada: Lahaye et al., 1995; Polat & eHfT 0). Kerrich, 2002; and eastern Dharwar, India: Dey, 2013) Crustal basement of the type represented by the > but lower eNd values in Neoarchean mantle-derived 2 9–3 7 Ga Narryer Terrane, if it extended to the study magmatic rocks are common (e.g. summary in Mole area at the time (e.g. Wyche et al., 2012), must be con- et al., 2018). The data suggest that the magmas, which sidered in interpretations of Nd isotope results for the formed the rocks studied here, interacted with older Norie and Polelle Group volcanic rocks. For example, of crustal components (crust or crustal melts, mantle the four rhyolitic and granitic samples with moderately 2160 Journal of Petrology, 2019, Vol. 60, No. 11
evolved eNdT (-2 2to-3 2), three contain inherited zircon which there is no correlation between the extent of in- xenocrysts with 207Pb/ 206Pb ages of 2950, 2960 and put and the resultant isotopic signature. The second is 3420 Ma (outlined in Fig. 15). The 2950–2960 xenocryst crustal contamination from old basement rocks corre- ages are equivalent to the older parts of the supercrus- sponding to the trend for xenocryst-bearing felsic sam- tal sequence, exposed along the southwest and western ples. The third type of contribution, which is observed margins of the Youanmi Terrane. The 3420 Ma xeno- in most mafic-intermediate rocks of the Norie and cryst age in sample 185939 (a single zircon grain; Polelle Groups, is derived from reserviors of intermedi-
Geological Survey of Western Australia, 2018a), which ate age, which allows Th/Nb and [La/Nd]PM to monitor is located 65 km to the west of our study area (Fig. 2), is the extent of its contribution to mafic magmas. consistent with the age of some granitic gneisses in the Based on isotopic ratios, zircon inheritance and Th/ Narryer Terrane (c.40 km west from that sample Yb-Nb/Yb systematics, the nature and extent of crust-
locality). mantle interaction progressively increased from low- Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021 Assuming a simple 2-component mixing system at levels of mixing in the Norie Group (c.2820–2800 Ma), to
2800 Ma, the boninite-like rocks with the lowest eNd moderate degrees of interaction in the Polelle Group (Bassetts Volcanic Member; eNd2800 to -0 5) would be (2799–2735 Ma). Th/Yb–Nb/Yb systematics and large modelled as a mix of 72–81% mantle component volumes of homogenized melt compositions observed
(eNd2800 þ3 9toþ2 1) and 19–28% of a model crustal in each volcanic unit require that the contamination pro- component (eNd2800 -11 5; Meeberrie Gneiss); the model cess occurred deep in the crust, lithospheric mantle, or proportions also depend on the Nd concentrations the mantle wedge. Crustal reprocessing is, however, assigned to the endmembers, but likely approximate likely to be a major factor for felsic rocks, as is demon- the maximum possible crustal contribution for the intra- strated by Yilgarn-wide whole-rock eNdT mapping of crustal contamination scenario. granites, which shows that two-stage depleted mantle One set of samples, dominated by high-Mg basaltic ages of 3000 to 3300 My are common (Champion, 2013; andesite to rhyolite series rocks of the Yaloginda and Smithies et al., 2018).
Greensleeves Formations, shows increasing [La/Nd]PM and SiO2 at constant Nd-isotopic ratios (eNdT 0toþ1; Subduction vs sagduction geodynamic models Fig. 15a and b). It is highly unlikely that samples plotting Consensus for a transition to geodynamic processes along these trends were affected by contamination of resembling modern subduction in the mid- to late old continental crust (requiring a contaminant with Archean appears to be growing and is based not only even higher [La/Nd]PM, SiO2 and eNdT 0toþ1), but the on lithological and geochemical arguments, but also on trend could reflect mantle enrichment by relatively metamorphic evidence (e.g. Brown & Johnson, 2018) young crust, such as the (2 92–2 98 Ga) lower supra- and detailed structural studies of the complex sequen- crustal rocks in the Youanmi Terrane. ces of events associated with orogens, often in ‘orogen- The remaining samples fall along a trend toward ic gold’ districts (Blewett et al., 2010; Bleeker, 2015). high [La/Nd]PM and SiO2 with decreasing eNdT (Fig. 15a Given that drip or ‘sagduction’ Archean geody- and b), extending to felsic rocks that contain xenocrystic namic models have a long history (e.g. Ayers & zircons, which could be perceived as a contamination Thurston, 1985, and references therein), they need to trend. However, a plot of Th/Nb vs eNdT (Fig. 15c) shows be considered in the context of our results. In most most geological units fall on a mixing curve involving cases, the sag or drip models are primarily concerned crust that is less isotopically evolved and, therefore, with cratonization where a stagnant lid or an existing younger (on average) than that which contaminated proto-craton is transformed into a long-lived terrane xenocryst-bearing felsic rocks. Samples at the more iso- through the formation of intra-crustal granite rocks. topically evolved end of the dominant Th/Nb-eNdT trend What the proposed sags or drips generally lack is a correspond to high-Mg basaltic andesites (Mg-number close spatial-temporal analogue for a developing man- of c.60) with pronounced Zr–Hf depletions (see ‘Mount tle wedge, although its absence may not be obvious Magnet East HMA’ of Wyman, 2019), which in in 2 D illustrations or computer models. In this regard, post-Archean subduction-related rocks are generally Archean rocks that resemble Phanerozoic proto-arc attributed to sediment melting under conditions where suites may provide key evidence for the viability of residual zircon remains behind (Elliott et al.,1997; sag/drip scenarios. Hermann & Rubatto, 2009). When these rocks and other The setting of the rocks studied here likely involved mafic-intermediate rocks from the southwestern existing evolved crust and includes rock types, such as Youanmi Terrane are also considered, it is clear that siliceous high-Mg basalts, that are not typical of samples with similar Mg-numbers (c.60) display a wide Phanerozoic juvenile arcs. Nonetheless, many of the range of eNdT on a near-vertical trend that does not ex- rock types observed in the northern Youanmi Terrane tend to the xenocryst-bearing felsic samples (Fig. 15d). can be viably amalgamated into a subduction scenario: Collectively, the isotopic data plotted in Fig. 15 show high-Mg basalts and picrites; moderately LREE- three types of non-mantle contributions. One is the pre- depleted tholeiites; and LOTI basalts, boninites and viously mentioned trend involving young crust, for related rocks from variably ‘re-enriched’ depleted Journal of Petrology, 2019, Vol. 60, No. 11 2161 Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021
Fig. 15. Integrated whole-rock chemistry and Nd-isotopic systematics. (a) eNdT vs [La/Nd]PM; (b) eNdT vs SiO2; (c) eNdT vs Th/Nb; (d) eNdT vs Mg-number; (e) Ba/Th vs Th; (f) eNdT vs Ba/Th. Transparent grey triangles in (e) indicate the compositions of northwestern Youanmi Terrane samples lacking isotopic data, other symbols as in Fig. 14. Outlier with eNdT þ2 0 relates to altered cumulate sam- ple 221754. Additional data sources: Windimurra and Narndee (Ivanic et al., 2015); Yaloginda Formation, Greensleeves Formation and Cullculli Suite (WACHEM database; GSWA, 2018 b); southwestern Youanmi Terrane (unpublished data). 2162 Journal of Petrology, 2019, Vol. 60, No. 11 mantle sources are recurring features of juvenile arc demonstrated by the Th/Yb vs Nb/Yb plots of the Norie systems (Bloomer et al., 1995; Meffre et al., 1996; and Polelle Groups (Fig. 13). Baziotis et al., 2017; Patriat et al., 2019). The later intermediate-felsic volcanic sequences can be equated Significance of textural observations from to more mature arc settings. In addition, the mantle hy- northwestern Youanmi Terrane mafic–ultramafic dration history associated with the Norie and Polelle ultramafic rocks Groups and contemporaneous mafic intrusive suites Lower Singleton Formation are consistent with the progressive influence of subduc- Mafic–ultramafic rocks of the Singleton Formation in tion zone processes. Sagduction models, in contrast, in- the Gabanintha area have previously been classified as volve the incorporation of water into melt pathways via komatiite and komatiitic basalt (Fig. 3; Jackson 1990; hydrated basaltic crust that sinks into the mantle. This Reudavey 1990; Watkins & Hickman 1990; Barley et al., process is a key requirement for intra-crustal granite Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021 2000; Hallberg, 2000; Pidgeon & Hallberg 2000; Van generation but does not provide mechanisms for sig- Kranendonk et al., 2013), but in each case, the classifica- nificant addition of water to the asthenosphere. Not tion was based on compositions of olivine cumulate only do models of descending drips of sinking crust fail rocks (i.e. picrites or olivine orthocumulate). Our data to create isolated depleted wedge mantle analogues, show that fine grained rocks (e.g. Fig. 4a and b), on the they are not capable of focussing metasomatic fluids other hand, mostly have basaltic compositions (SiO into evolving boninitic source regions. These are gener- 2 49 9–52 7 wt % and MgO 10 3–16 1 wt %), whereas sam- ic weaknesses of sagduction models, and our results ples containing 18 wt % MgO all contain abundant provide additional specific reasons to adopt an alterna- olivine phenocrysts (e.g. Fig. 4c–f) and, therefore, do tive model. not represent liquid compositions. Furthermore, several of those earlier studies identified textural features un- common for komatiites, including abundant amygdales Nature of mantle metasomatism in both fine grained and picritic rocks, and a pyroclastic The samples for which e are available are plotted in NdT component within the volcanic sequence (Reudavey Fig. 15e on the Ba/Th vs Th plot of Hawkesworth et al. 1990; Barley et al., 2000; Hallberg, 2000). Both of those (1997), where they display a similar distribution to that features indicate a relatively high volatile content and observed in island arc settings. Most geological units are consistent with a hydrous mantle origin. fall on distinct trends, where multiple samples are avail- able. A trend to high Ba/Th at low Th is defined by the Narndee and Windimurra Igneous Complexes, Boninite-like rocks Singleton Formation (basalts and picrites, boninite-like Volcanic rocks of the upper Singleton Formation show rocks and variably LREE-enriched tholeiites), and textural evidence for magma mixing (or rather, incom- Meekatharra Formation (tholeiitic basalts, siliceous plete mixing), such as rafts of more mafic groundmass high-Mg basalts and boninite-like rocks). This trend cor- ‘intruding’ slightly lighter bulk-rock groundmass responds to the fluid-related mantle metasomatism (Fig. 5b), and mafic globules in leucocratic domains, trend documented by Hawkesworth et al. (1997) for the within an otherwise mafic groundmass (Fig. 5e and f). Marianas, Tonga, and some other intra-oceanic arcs. Alternatively, the mafic globules could reflect immisci- A second trend at low Ba/Th to high Th is defined by bile domains of a solute-bearing hydrous fluid exsolved basaltic andesite-rhyolite samples of the Norie and from the silicate liquid–crystal mush before or during Polelle Groups (including adakitic compositions) and emplacement, as proposed for globular/orbicular fea- similar aged high-Mg andesites from the southwestern tures by Ballhaus et al. (2015). Such a mechanism could Youanmi Terrane (assigned to the Meekatharra also be responsible for the common occurrence of ‘vari- Formation; Geological Survey of Western Australia, oles’ in boninite-like rocks and siliceous high-Mg 2018c). Hawkesworth et al. attribute this latter trend in basalts of the Meekatharra Formation (Fig. 7). Boninite- the Aeolian Islands and the Philippines to sediment- like rocks in the Bassetts Volcanic Member related mantle enrichment. Northwestern Youanmi (Meekatharra Formation) are typically vitrophyric or Terrane samples that fall along this trend also show microlitic textured, like many Cenozoic boninites (e.g. many of the classic features of sediment-related mantle Falloon & Crawford, 1991; Taylor et al., 1994), with enrichment, such as strong Th-LREE enrichment and zoned pigeonite–augite phenocrysts (Supplementary negative or positive Zr–Hf anomalies (related to residual Data Electronic Appendix 3). Locally, the Bassetts zircon or zircon melting in the source respectively) and, Volcanic Member also exhibits dendritic pyroxene and therefore, plausibly represent a classic ‘sediment’ trend platy pyroxene spinifex textures, which have rarely in an arc mantle wedge setting. In a geochronological been described in the literature (see discussion in context, these results collectively imply that a switch Lowrey et al., 2017), but have been reported in one from fluid- to sediment modified sources occurred twice Cenozoic boninite from New Zealand (Wood, 1980). within the studied Norie–Polelle sequence (Fig. 15 and Compositions in the freshest cores of zoned clinopyrox- Table 3) and was associated with two transitions from enes range from pigeonite to clinoenstatite depleted to metasomatised mantle, as also (Wo4En83Fs13 to Wo15En68Fs17; Supplementary Data Journal of Petrology, 2019, Vol. 60, No. 11 2163
Electronic Appendix 3, Supplementary Data Fig S3.2), geochemical trends observed, we favour a subduction- and in cross polarized light they show tightly spaced like process for Norie and Polelle magmatism, although polysynthetic twinning along the c-axis (Fig. 8f). the mode of Archean subduction may have been differ- Polysynthetic twinning in clinoenstatite from Phanero- ent to the styles that dominate in the Phanerozoic (e.g. zoic boninites has been attributed to stacking disorder Perchuk et al., 2019). Subduction of relatively young arising from the transition of protoenstatite to clinoen- crust at c.2800 Ma resulted in hydrous metasomatism of statite during rapid cooling (Dallwitz et al., 1966; mantle sources and modified the LREE–MREE charac- Komatsu 1980). Another consideration is the transition teristics of many magmas without significantly influenc- from pigeonite (–clinoenstatite) cores to augite rims, ing their Nd isotopic signatures. Sediments, with which contain unusually high concentrations of the in- variably evolved isotopic ratios, were increasingly im- compatible minor elements Al and Ti; an early study of portant in the genesis of intermediate to felsic magmas
lunar basalts by Grove & Bence (1977) showed that the during both Norie and Polelle Group volcanism, al- Downloaded from https://academic.oup.com/petrology/article/60/11/2131/5700738 by guest on 27 September 2021 pyroxene-liquid partitioning behaviour of Al and Ti was though mixed fluid-sediment contributions appear to strongly dependent on the cooling rate. In our samples, have also occurred. The fluid-to-sediment re-setting of rapid crystallization of Al–Ti-poor pigeonite cores likely metasomatic sources can be accommodated in a var- resulted in correspondingly Al–Ti-rich halos surround- iety of subduction-linked scenarios: ‘short-lived’ Norie ing the cores: the higher than normal Al and Ti concen- Group subduction (c.20 million years) may have been trations in augite rims, therefore, re-enforce a high terminated by a reconfiguration of plates, such as in a cooling rate model. subduction zone step back, which also initiated Polelle All textures observed in boninite-like rocks and sili- Group magmatism. ceous high-Mg basalts in our study area indicate rapid Post c.2780 Ma, we identify a volcanic hiatus in our crystallization and undercooling, to varying degrees, study area of up to 20 Myr, which separates the during emplacement (Lowrey et al., 2017, and referen- Meekatharra Formation from the Greensleeves ces within). Recognition that variolitic or ‘globular’ tex- Formation, but its tectonic significance is not yet clear tures can result from immiscibility between exsolved and may be a local feature within a larger scale subduc- hydrous fluids, and silicate magma (Ballhaus et al., tion system. 2015), is petrogenetically significant for the high-Mg mafic volcanic rocks in our study area, since it requires an H2O-rich parent magma. Considered together with SUMMARY AND CONCLUSIONS the primitive compositions seen in our data, the petro- Boninitic and LOTI basalt suites occur in many Archean graphic evidence indicates that the lavas were very hot, cratons and have played an important role in assess- phenocryst poor, and likely wet during emplacement, ments of tectonic styles operating during Earth’s early which in turn indicates a rapid ascent through the crust history. The 2820–2735 Ma northwestern Youanmi and reaffirms that crustal assimilation prior to eruption Terrane stratigraphy contains perhaps the best pre- was minimal. served and most complete set of rock types that would be considered diagnostic of subduction magmatism in the modern geologic record (boninite, LOTI basalt, tho- Geodynamic synthesis leiitic basalt, high-Mg basalt–andesite–dacite, sanuki- The Th/Yb–Nb/Yb trends in c.2800 Ma northwestern toid and hornblende-rich gabbros). Youanmi Terrane rocks indicate that mixing between Using a combination of new whole-rock composi- crustal and mantle components occurred either in the tions and Sm–Nd isotopic data we have demonstrated mantle, or in mixing zones within the lower crust, be- that: fore the magmas ascended to surface, otherwise con- tamination trends would be apparent. This is supported 1. Previously reported Al-depleted komatiites in the by whole-rock Sm–Nd and zircon Lu–Hf data (this study; lower Singleton Formation are instead high-Mg Ivanic et al., 2012, 2015), which predominantly show a basalts, picrites and olivine orthocumulates. Wyman trend towards increasingly LILE–LREE enriched compo- (2019) noted their similarities to Phanerozoic arc- sitions at near-constant (eNdT -0 5toþ1 5) isotopic picrites in terms of magma compositions and field ratios. Together with contemporaneous hydrous characteristics. Nonetheless, the rocks also bear a mantle-related magmatism of the 2799 6 7Ma strong similarity to Barberton Al-depleted komatiitic Boodanoo and 2801 to 2793 Ma Warriedar Suites rocks with respect to MgO concentration, trace elem- (Ivanic et al., 2015; Ivanic, 2016), the evidence favours a ent patterns and incompatible element ratios (e.g. subduction origin or another process that might repli- Al2O3/TiO2). Their emplacement at the lowest strati- cate key features of the subduction process. graphic level leaves open the possibility that the first Based on the recurrence of multiple rocks types in cycle of subduction magmatism in the Youanmi re- the northwestern Youanmi Terrane that commonly gion at c.2820 Ma was triggered by the ascent of a occur in Phanerozoic arcs, the geochemical evidence nearby mantle plume. for two-fold mantle depletion and re-enrichment associ- 2. High-Mg basalts and picrites of the lower Singleton ated with mantle hydration and the distinct isotopic- Formation and tholeiitic basalts in the Stockyard 2164 Journal of Petrology, 2019, Vol. 60, No. 11
Basalt Member erupted with minimal interaction Survey of Western Australia. JRL received support from with crust, which in turn indicates locally thin an Australian Government Research Training Program ned lithosphere and upwelling asthenosphere, con- Scholarship. sistent with two episodes of rifting at c.2820 Ga and sometime between 2799–2787 Ma. 3. Th/Yb–Nb/Yb systematics and Nd isotopes suggest LILE6LREE enrichment was a characteristic of their SUPPLEMENTARY DATA mantle source, which evolved in composition over Supplementary data are available at Journal of time. Petrology online. 4. The mantle sources of boninite-like rocks in the upper Singleton Formation and Meekatharra
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